It never ceases to amaze me how many people do not enough or no research before doing something like overwriting their device's ROM.
Like Rafy said -- try flashing your carrier's stock ROM. Can you get to the bootloader (tri-color RGB screen)? Press volume down and press the reset button with the stylus at the same time.
If you can get that far, you can reflash stock. Plug phone into USB, wait for it to say USB at the bottom of the bootloader screen on the device, and then start the stock ROM installer which can be found at the link CovKid supplied you.
http://forum.ppcgeeks.com/showthread.php?t=91940