This one is easy. Download a ROM you want on there. If you need a stock one, I suppose you didn't make a stock NANDROID backup then go here:
Stock ROM | incredible ROMS
1.Place the .zip file onto the root of your SD card, not in a folder etc.
2.Turn your phone off fully. Hold power, power off.
3.Hold volume down and hit power, do not let go.
4.Once in hboot hit volume down to highlight recovery. (It will take 10-20 seconds before it will let you move it.) Power button to select.
5.Go into clockwork mod, click install from zip from sdcard. Press optical button to select.
6.Find your zip select it with optical track ball.
7.Let it do it's thing, you have a working phone again.
8.Start reading BEFORE you do anything else.
9.When you are tired of reading create a nandroid backup with clockworkmod.
10.Don't do what you did before twice.
This process only works if you didn't flash a 2.x radio for a Froyo ROM. If you did instead of a stock ROM I would suggest you download Skyraider 2.5.2:
http://forum.xda-developers.com/showthread.php?t=722481
Good luck!