First suggestion is to provide a better narrative. If I had to guess from the very limited description, I would guess that your backup software wiped whatever files were on the disk. But let's try for more detail
What OS are you running? Is there only one computer involved in this story?
First you just used it as a drive letter and wrote files to it, yes? At this point, could you see the files if you came back the next day and attached the drive to the computer?
Second, you used it for backup. What backup tool did you use? Did you point it's storage to a new directory on the drive, the entire drive, or what?
When you write that now you can't access anything, can you read the backups with the backup program, or not even those? If nothing at all, is the drive detected in Disk Management, Device Management, and/or BIOS?
I wish that they would change my title to "Compulsive Disambiguator."