Just reread your first post, initially pressure was shooting up to 50 psi and now 20 psi? That might be a good sign.
One side of the reg is connected to the full output pressure of the Co2 bottle, 500-600 psi and is read on the the high pressure gage directly opposite the input.
The top gage is the regulator adjusted output pressure. The pressure adjustment screw on the reg controls a spring loaded "valve" that will maintain set pressure. Normally, as the adjustment screw is turned clockwise the regulated output pressure increases. The pressure you dial in, shown on the regulator out gage is the same as is going into the keg. The seal that controls this valve this is is made of rubber and susceptible to foreign object damage.. Beer.
If you had carbonated beer before you ran out of gas the pressure in the keg was probably a little higher than the pressure in the CO2 tank, (CO2 tanks seem to go from 500 psi to zero quickly) which could beer to be pushed into the CO2 line, into the regulator and damage the seals on the "valve".
Beer push back into CO2 lines is why many people install check valves at their regulators or CO2 distribution manifolds.
If you have a one way check valve(s) protecting from this, then still sounds like the reg broke on you.
I'm dissappointed more people didn't post. Lot of keggers on here. Hopefully more people will post..