Shower is either all hot or all cold
It looks like you have an issue with the mixing valve which looks like this -

More specifically it is probably due to an issue in the cartridge which is the front part of the mixing valve and is usually a white plastic cylinder that look like this -

These are in the wall right behind the shower handle.   It is likely that something is either jammed up (sediment) or broken in the cartridge.   If you open things up and take pictures we can help more.