Hi tobyhewitt,
Almost certainly an aerial problem.
What kind of aerial does your car have?
A few thoughts depending on the answer to the above question:
If you have a screen/heated rear window type aerial, make sure power is being supplied to the aerial (this is because these often use an amplifier to boost the signal) and without power you'll hardly get any reception.
If you have a normal body-mounted aerial, a few things you could check: first that it is plugged in securely at the rear of your stereo, secondly that it is earthed adequately to the bodywork of your car, and thirdly that the aerial is not shorting out with the body of the car (can happen for example if the aerial lead has become trapped and the insulation is severed).
If you're uncomfortable with checking these things yourself, I'd certainly recommend finding a friendly local car radio shop. I think it's pretty unlikely that the stereo is at fault if everything else is working fine, so should be quite quick to put right I would guess.
Hope this helps.
