Accidentally formatted an exFAT partition to OS X journaled and lost data
If you haven't done anything else to the disk besides reformatting it, if you haven't copied any other data to the drive, then you will likely be able to get most if not all of the data back using a data recovery tool.

However, because the drive's original format was a PC format, you'll want to use a PC data recovery tool, not a Mac one. Something that can recover deleted files. You may get a messy jumble of pervious versions of saved files too, that's just the nature of recovering deleted files. 

On the Mac, something like DataRescue works very well. But you need to do this on a PC to read the PC formatted drive.