There are two fuses for the radio - pull them both. (Sorry, but the exact fuse varies by year, I found this out the hard way this summer). Both fuses are in the undershood fuse box, driver's side. Leave them out 15 seconds or more.

Alternatively, disconnect the battery ground cable for 1/2 minute.

When you power back up, it should restore it's brains. If not, it is done for - sorry. But many on this forum have upgraded to way better head units (myself for one!) and have the OEM radio sitting on a shelf in the basement. If you look, you CAN find one.

But do not overlook the idea of just upgrading to something newer. The Kenwood/JVC (and I think Clarion) marine radios are the same color as our dash and they sound WONDERFUL compared to the factory unit.

I put a JVC 660 in my car (it is discontinued), $80 at Best Buy and WOW did is enhance the sound in my car.
I had it wired up and installed in 1/2 hour.