Tell you what happened to me. Was involved in a rear-end collision with an 18-yr-old. Popped *everything* out of the dashboard, including my JVC CD Receiver. Simply put everything back in its place; no harm, no foul, right? Uh-uh.
My receiver now had a really hard time picking up FM stations, and wouldn't receive AM stations at all, despite the CD player working perfectly. Turns out that the antenna cable connecting the receiver to the car's antenna was broken. That is a nasty fix.
My advice: ask around to find a good car stereo place, and have them check the cable. Alternatively, you could try to remove the antenna base, attach a string to the end of your cable, tie the other end securely to a new cable (which cable you need depends on your make/model and the receiver), and *carefully* pull through to replace. If it turns out that the cable is secured in places (most are), you'll need to remove the dash and visually trace the cable. **STRONGLY** suggest you consult a pro unless you are comfortable tearing up (and putting back together) your car's interior.
PencilSharp!, August 2005