I hope I explain this properly:
The felt, or whatever material the interior is made of, on my car’s ceiling is starting to pull away from its backing and is caving in. The car is pretty old, a '95 Saturn, but it runs great, 35 MPG, and I would like to keep it a few more years. I need to know if there is anyway to get that to stick back to the ceiling or if I need to replace the whole thing. If I do need to replace it any idea on the costs? Thanks in advance.
I had this happen years and years ago with a Celica. I simply got some spray adhesive at the auto parts store and carefully applied it between the sagging cloth and the roof, carefull not to leave bubbles or miss any spots. Also, make sure it dries completely before you drive around with the windows down or the buffeting by the air will cause it to unattach again.
I had an identical problem on an Oldsmobile I used to own. I found a solution, but it can be tricky.
Get some spray glue. The canister must contain one of those nozzle extension straws (like the ones that come on cans of WD-40).
Take a razor knife and cut a tiny incision in the felt. Start in the back of the car where it is not as likely to be noticed if your first attempt doesn’t go that well.
Put the nozzle through the incision, and let spray the glue. This must be done quite sparingly, however. If your ceiling is anything like mine was, using more than a light spray will cause the glue to instantly soak through the felt, causing quite a mess. Just use quick shots of spray. You’re trying to get it on the ceiling, not on the felt.
Press the felt firmly onto the ceiling.
Repeat wherever needed.
If the felt is coming off of the entire ceiling, then keep in mind it is going to be stretched out. Once you glue it all back up, there will be leftover felt that can wrinkle and bunch up. To help avoid this, begin the in rear center of the ceiling. The goal is to have any leftover stretched felt end up on the sides of the car, where they can be tucked away or generally made less noticeable.
Hope this helps!
D’oh! A preview shows that lieu beat me to this. Well, I’ll just second what he said, then. 
Be sure you get “Headliner Adhesive” and not any other kind of glue. It has to be able to keep its bond in high heat. Before you use the glue, check to see if you can loosen the trim pieces at the edges of the headliner. Then you’ll be able to apply the glue, press the headliner back up, tuck its edges under the trim, and screw the trim back down on it, holding it tight. If the whole headliner is coming down, it helps to have an extra pair of hands or two for this procedure.
Then, next summer when the whole thing collapses on you again, you’ll know what to do.
You can get it fixed professionally for best results - they reupholster your whole car ceiling, I think. When I needed it in 1997 or so it cost about 400 bucks or so but was very nice as when I bought the car the ceiling upholstery was in quite the advanced state of slump and touched my head with a bit of slack while driving. (And I wasn’t really all that tall either.) This was in an '88 Buick Park Avenue, FWIW.
The piece of cloth is just glued to a piece of cardboard or foam that fits in the ceiling and is held up by just a few clips or sometimes just the side pieces of trim. For the best results I’d take the side trim off, pull the entire piece of backing out. Use 3M Super Trim Adhesive, it comes in a spray can, spray both the backing and the new cloth. (I’d get new cloth, it’s much easier to work with than trying to line the old one back up). After you spray both sides with the adhesive, press it into place starting on one corner. Cut out the holes for the dome light, etc… put it back in place, pop the clips or the side trim back in and you’re done. About a two hour job max, and it will save you about 350 bucks.
I inherited a POS car from the ex where this was happening. Back in 1997 or so I did a quick fix with some staples until I could get around to the more permanent solutions. The staples were still in it in 2000 when I sold the car.
spring for re-upholstry. I did the spray glue (correct kind, applied in various ways), I did staples (I was mad, it was ugly but it worked) but ultimately a full re-do is the best way I’ve found to do it. I’d do staples for my ‘new’ car except for the toddler who rides around in the car all the time with us now …
Amp, I got mine reupholstered for about $80 maybe 5 years ago at this little hole in the wall in Pinellas Park. They did it while I waited; it took maybe an hour. It’s still holding strong. If you want the name of the place, let me know, and I’ll go find it for you.