If you have any valuable data /"I have all my info from 2003 on this"/ on this disk you by any means should NOT format it.
+ If you can't read it on a PC too it's likely not a format issue.
It may be caused by the enclosure electroniks, and in case disk itself is OK you may pull it out of the case and move it to another case or a docking station. Or you may get the bare drive and attach it internally in a desktop.
I repete: under any circumstances DO NOT FORMAT this disk!