OK, first it is important to understand that we are living through a transition in hard drive tech. Any drives larger than 2TB will require some different thinking.
The MBR file system that we have been used to for years has a limitation. For technical reasons that system cannot be used on drives or partitions larger than 2.2TB. For drives or partitions larger than 2.2TB you must use the newer GPT file system.
In addition to that it is important for everyone to know that many older hard drive enclosures cannot read drives larger than 2.2TB. So in your case it might be as simple as you need a newer hard drive enclosure.
As for your data, changing the file system format can cause you to lose your files. The good news is that all the data is still there on the disk, but the new file system hasn't "put it all together".
The first thing I would do is put the drive back in the desktop and see if you can see your files there.
If not then you could look at a free program like RECUVA to use to recover your files.
Let us know what you find.