Apart from the launch infrastructure and skilled labor issues brought up by previous posters, one of the significant shortfalls would be propellants. While the cryogenic propellants for the main engines are easily manufactured, the propellant grade ammonium perchlorate oxidizer, granularized aluminum fuel, and polybutadiene acrylonitrile (PBAN) binder may not be readily available in sufficient quantity to produce even two Solid Rocket Boosters (SRB). Also, given that ATK has shut down the SRB production line with the defunding of the Ares I CLV and Ares V HLV boosters, a new production run would require requalification including sample testing and a static fire of at least a motor section. There are doubtless other material and component obsolescence issues as well with things like insulating tiles or avionics, although these could potentially be scavenged from other vehicles in the fleet for a one-off mission.
Although the External Tank (ET) production line was shut down, I suspect most of the tooling fixtures are still intact as both the Ares V core stage and several versions of the proposed Space Launch System use the current ET as the basis for those vehicles. I suspect an ET could be produced in 9-12 month time, assuming availability of the AL 2195 material (an aluminum-lithium alloy that was fairly exotic when first introduced but is not becoming increasingly used on commercial aircraft applications).
As for going to the Moon, neither the United States nor any other nation has a launch vehicle (much less a man-rated one) that is capable of lifting an Apollo-sized capsule with an LM and a trans-lunar injection (TLI) stage like the S-IVB into orbit. The proposed SLS could do this and would use predominantly Shuttle-based hardware, but even with an accelerated test program it is difficult to postulate this being implemented in less than 6-8 years. It is true that the Saturn V was developed in a shorter period, but this was a blank check crash effort, and quite frankly the Saturn V would not meet modern man-rating standards (though it would come closer than the STS). An alternative is multiple launches of an existing heavy launch vehicle like the Delta IV Heavy with an Earth Orbit Rendezvous of the capsule, lander, and translunar injection stage, but logistically this would be highly complex (especially as there are not three facilities capable of launching the D-IVH to the same azimuth).
There are alternatives, however. The Russian Energia rocket consisted of a core stage which used the RD-0120 LOX/LH2 engine and four or more RD-170 LOX/RP-1 Stage 0 boosters. The RD-0120, which is similar in designa and performance to but less complex than the SSME, is sadly out of production, but it could be easily replaced by the Rocketdyne RS-68 that is currently used on the Delta IV with only a very slight hit on performance. The RD-170 is still built in a slightly uprated version as the RD-171 in the Zenit SLV. Better yet, the Energia was quite modular owning to the throttleable liquid boosters, and so the vehicle can be scaled for different reference missions by adding or subtracting boosters and throttle profiles. Given that the propulsion engine systems are mature and robust (and most importantly, already man-rated0, it is conceivable to develop and test an Energia-like rocket in a 3-5 year timeframe.
Even better would be to go toward a simple, robust system like the Truax-designed Sea Dragon. The Sea Dragon traded high performance in favor of a large payload, simple pressure-fed propellants, and best of all, minimal infrastructure. Because it was floated out to sea and erected by ballast, it did not require expensive ground facilities, could be launched from a wide range of azimuths, could perform multiple sequential simultaneous launches, and otherwise simplified the logistical problems that plagued the STS. Truax didn’t intend the Sea Dragon to be man-rated but there is no reason that it could not be, especially since it would be feasible to add multiple redundant systems without a substantial weight penalty and it would be relatively simple to undergo an extensive launch test program at low cost with reusable assets. Truax actually tested a proof-of-concept vehicle called the Sea Bee, recovering and refurbishing for a total of seven launches. A modern version could take advantage of improved propulsion system that would allow near-optimal performance at both sea level and altitude, perhaps even leveraging off of a larger version of the aerospike engine that Rocketdyne designed and tested as an alternative to the complex ultra-high pressure staged combustion SSME design that was ultimately forced upon them by NASA. I would conservatively estimate that this could be put into production in 6-8 years, and perhaps practicable in 4-5 with sufficient impetus and resources…provided, of course, that the current prime contractor isn’t placed in charge.
None of this accounts for the TLI stage, although I’ve heard some rumblings about bringing the Agena back on production. (I think that’s wishful thinking, but I could be mistaken.) An enlarged Agena might be capable of serving as a TLI stage, or we could rebuild a S-IVB-type design using the J-2X or a vacuum-optimized RS-68. There is currently no lander design but that is less challenging than the CSM and launch vehicle in terms of capability. The biggest problem is what you are going to do on the surface. As we discovered in the later extended duration J-missions, the problem of lunar dust was an enormous hindrance and potentially even a safety hazard. Even longer missions or more extensive efforts would require some novel technologies and techniques to cope with the electrostatically charged powdered regolith.
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