You broke the stop in the cartridge. The only thing to do now is drill out the head of the screw. If you use the right size, you should be able to drill out the head and get the handle off. There should be enough of the screw left that you can remove it with pliers, but you will probably be replacing the cartridge. They are not terribly expensive. The most expensive is around $30. Drill bit size is probably 3/16 or 1/4 if you are careful.
No conversion, it would need to be replaced with a single handle faucet.
Generally, the handle needs to be removed either by popping a single tab off the center of the handle, or removing a set screw from the side. Once the handle is removed, there will be a plastic ring through which the center post connects the handle to the valve. This black ring and be pulled out, and rotated to raise or lower the maximum temperature setting.
Temperature control using a single lever handle is easy, the handle only turns a maximum of 90 degrees. one side cold, one side hot and the middle a mixture of both turning it slightly either way will increase whichever side is more open
Moen part #1920 and a brand new Moen single handle faucet
Look under the handle for a small ALLEN SCREW (hex head). Loosen it and slide the handle off.
First off - be on the outside of the shower and screw with it clockwise and anticlockwise and see what happens. Works every time.
Sounds like you hooked up the the supply lines backwards .
need universal handles for Bath & Shower, they will replace old style American Standard, need a handle with an adapter to fit different size stems. Preferable modern style handles if possible
On single handle shower valves, designed to let cold water first and turn handle farther to mix hot water to desired setting. If hot water turned on first, possibility of getting scalded. If seperate hot/cold handles for shower, shower valve needs work.
3/4 turn
Most likely the inside guts of the cartridge area are all broken now. I have removed the rest with EZ Outs. It pushes in and you turn it counterclockwise and hopefully it will release whats left. Just make sure to try and not damage the brass side walls.
The handle must be removed to allow access to the retainer clip, then the cartridge pulls out. You may need a cartridge puller if the faucet is old.