What OS are you running? It's rare that the system WANTS to format, sure you can't just click out of there? If not can you access the drive through explorer (just hit your windows key+E) and copy the files somewhere safe? Otherwise, try power cycling the PC. If it really does want to format and there aren't any other choices, a power cycle should revert back to the old drive letter and you can start over. (A power cycle is holding in the power button on the PC till it shuts off, not good for the hard drive but better than losing data). Good luck!
-Chris