Is the pin/ball depressor still on your fitting? Have you cleaned your lines taps? One way to eliminate the lines and tap is to depress the lock with something clean and if beer flows then its your lines or tap. If nothing happens you have a CO2 issue. Hope this helps.
when this happened to me it was the poppet in the ball lock fitting of the keg. i figured that out because the keg would not dispense on any tap or even the picnic cobra tap.
YMMV. there can be several problems with this symptom.
The poppet is inside the fittings on the keg. Your LHBS should have a socket to remove these. It is the part that is essentially a spring loaded valve. I made my removal tool for my pin locks by dremeling slits in a spark plug socket. You then pry them out and replace with new. Again LHBS should have these. Midwest does also.
Also need to make sure the valve is open from the regulator if you have a valve. You can show whatever PSI and a full tank, but if the valve isn't open, you aren't actually getting any Co2 to the keg itself.