Inside that cartridge, there is often a pressure balancing device. Basically what it does is if one input suddenly depressurizes, it softens the shock of the change for the person in the shower. This is a rudimentary explanation, but should suffice for our purposes here.
I had a valve (brand new) once that this device had gotten stuck to one side for some reason. I got the result you're having... only a tiny trickle from one temp or the other, can't remember which.
If you feel up to the task, you can take that valve cartridge apart a bit more, and find this balancer. You'll want to free it up, and make sure the lubrication on it is still there so that it can move freely. This should fix your problem. If not, the cartridge is probably $20-25, and you're done. I can't identify it from the picture, but I bet someone on here can.
The sudden decrease and increase of pressure on the lines could have caused this with the water mains being shut off/turned on. They also could have had a surge in the pressure or something while working.