That method of shutting a machine down isn’t particularly recommended unless the machine is hung, but it shouldn’t hurt it to the point of not booting, either.
First, check to make sure that there’s no CD or floppy in the drive; it may be trying to boot from that.
Second, check the BIOS screen (probably F1, F2, or DEL while the machine is booting) to make sure that the boot device is set correctly to the primary hard drive.
Third, let it sit at the blinky-prompt for a good hour or so; if it’s cycling through boot blocks, it can sometimes get hung for a surprisingly long time.
Fourth, boot from the XP install CD. When it asks you what you want to do, hit “R” to use the repair console. If it can find a disk, it will ask you for the admin password. Enter it, then type “CHKDSK /F C:”. If it whines at you about not being able to find AUTOCHK.EXE, skip this step and go on (it depends on the exact settings of your boot disk).
Fifth, get back to the repair console, then type “FIXMBR” at the prompt (say yes when it asks you if you’re sure). Note that at this point we’re into the fairly desparate options. FIXMBR is usually safe, but CAN mess up the partition table, at which point you’re toast.
Sixth, do the same thing, except add “FIXBOOT” at the prompt before “FIXMBR”. We’re really grasping at straws here.
Last, you’ll have to restore the drive. If you’ve got something like Norton Utilities (which usually does more harm than good), try it as step 4.5