Replying to Computer not recognizing external hard drive.

Seagate Free Agent Hard Drive, 1.5 TB. Plugged in using USB. Im running Windows Vista. I was using the drive earlier this morning... Un plugged it to plug into a friends computer, never plugged into any other computer and I plugged it back into my computer, now I can't access the drive and my computer wants to format it... I have about 700 gigs of files on there that I can't really replace. Anyone have any suggestions?