I backed up my iPhone and now I can't find the backup?

I was unlocking my phone and backed up my iPhone 5 to the computer not to the cloud. Then to complete the unlock process you have to master erase your phone. After I did that I went back to restore my phone and the backup wasn't there! I know it has to be somewhere cause the backup too 20 mn of my time so I hope it's somewhere please help

When you back up the iPhone you normally do it via the iTunes. So check your iTunes for the most recent back up.
Now if you over zealously "master erased" your iPhone and did not pay attention to what you were doing then you may have accidentally erased this back up as well!
In this case you are in trouble unfortunately.