I've had my Series D working fine, or as fine as I could figure. It was hard to roll past tension 4. Anyway, Suddenly when I fire the camera only the mirror goes up and the camera doesn't fire. What may have happened and is it fixable?

It seems to have started working again. I'm not too sure what I did to make it fire again. Maybe it needs oil? I don't know the first thing about servicing these. So any help would be greatly appreciated. I'm very hesitant to open any of these old cameras. I feel like I turn a screw and springs pop out hahah!

Refer to the online Graflex SLR operating instructions http://graflex.org/RBGraflex/

Verify that the I/T lever is at the I position, T fully covered.
Look at the base of the M lever, it should be straight. I've encountered heavily used cameras that had the lever rounded out and it would not lift up far enough to to release the shutter. Is the pin at the I position 90° to the I/T bar?

Set the tension before winding the curtain to the desired aperture. Run the curtain down to O or closed after O for easier tension setting.

For servicing, I send my Graflexes to the Camera Doctor in NYC...

http://www.cameradoctornyc.com/

The guys there are great, and will give you an estimate before doing the repair.