I am assuming that nothing else changed when this problem began, that the camera was appearing on the desktop/in the file manager one day and not the next day.
Wear any updates applied or any software installed or removed?
The only time I've had a similar problem is with a camera with an MFT (I think that's what it's called) file system; I had to install gtkam to connect to it, but it never connected simply as a mass storage device.
Here's two thoughts for testing:
Try connecting the camera to another computer to verify that the camera still will connect; if you don't have one, maybe one one belonging to a friend or, if you are lucky enough to have access to a LUG, one belonging to a LUG member, or even a Windows box. It could be that the problem is with the camera, not with the computer. It might also be worthwhile to test the SD card separately, assuming is has one, just to be thorough.
After that, try connecting to your computer and then checking the logs for any errors. They would likely appear in /var/log/messages and the tail command would be the most efficient way to check, something like
tail -50 /var/log/messages
See man tail
Sorry I can't offer more, but maybe this will be a start.