first off pull the battery to reboot. Hopefully it will boot up fine from here (back to pre-OTA state). Then check for software again, it should redownload the update.
Note: if you are rooted, unlocked, or on non-stock hboot the OTA will not work. I have standard Hboot, but was unlocked and rooted. So to run the OTA I had to relock and run the RUU.