Do you have a custom recovery? If so, if you made a backup using it, restore that. If no backup you should be able to use adb while in recovery mode to push a new copy of build.prop if you can find one, or try copying your build.prop.bak to build.prop and then rebooting. And if none of those work then you could try flashing a ROM.
If you don't have a custom recovery (and personally I wouldn't touch anything in system without one) then you may need to reflash the phone - the stock recovery won't let you run adb, so if you can't boot there may be no alternative.