Hey there, xander m.
Unfortunately, you can't "unformat". You might be able to use a partition recovery tool to try and recover the partition, but I'd recommend against it, because this could lead to damaging the data. When you reformat, you just change the file system of the drive, but the data is still there. The file system is needed by the OS in order to be able to access the files on that partition and to "know" where everything is. Since you've changed it, it looks like there's nothing on the drive, but all the data is actually still there. It's very important not to write anything on that drive, as the new data will overwrite some of your old data and make some of your old files corrupted and irretrievable. In your case...