Start by checking that the aerial hasn't been broken off!
If you're confident that the aerial is able to do its job, there could be something within your car that's causing electrical interference, which is blocking the signal. Try turning the radio on without turning the ignition key (if your car allows that to happen). If the radio works fine at that stage, but plays up when the ignition key is turned to the 'accessory' position (and/or to 'on'), you'll need to track down the interference.
I had a Citroen BX where the radio worked perfectly with the ignition key in the 'off' position but got nothing but 'static and noise' (as in your post) as soon as the key was turned to the first ('accessory') position. (i.e. without actually starting the engine). I eventually tracked down the fault by removing one fuse at a time until the fault disappeared. It turned out that there was a shorting connection inside the motor for the rear windscreen wiper (which isn't exactly the first place that most people would look for a radio fault!).
So the cause of your problem could be almost anywhere inside your car, meaning that some detective work from you might be necessary!