My car will not stay on after changing the belt why?

First make sure all electrical connections are tight and no wires, including grounds, have been damaged during the other work you did. Then make sure your battery is fully charged. After that I would load test the battery to make sure its still good. Then scan the car for trouble codes. The comuter may know what's wrong and is just waiting to be asked. Checking the fuel pressure to make sure the fuel pump is doing its job wouldn't hurt either.

Check that you ran the serpentine belt the correct way around the AC sprocket. There should be a diagram somewhere on the fan cover,