Sometimes even new(er) valves fail. If you have hard water then that is plenty of time to mess one up. In a best case scenario, you can lookup your make/model of faucet to buy a ‘cartridge’ for it and find the instructions on how to replace that. In more extreme cases / poor designs, you could have to tear into the wall and replace the entire thing.
You might also see if you can find a plumbing community and post as many details as you can… I’m an experienced DIY person who has done a lot hands on and read a lot, but I’m not a plumber so I still have blind-spots.
A lot of showers need a new mixer valve…
I have this issue in a home built just barely pre-lockdown. So the issue isn’t wear and tear.
What should I be looking for to fix this?
Sometimes even new(er) valves fail. If you have hard water then that is plenty of time to mess one up. In a best case scenario, you can lookup your make/model of faucet to buy a ‘cartridge’ for it and find the instructions on how to replace that. In more extreme cases / poor designs, you could have to tear into the wall and replace the entire thing.
You might also see if you can find a plumbing community and post as many details as you can… I’m an experienced DIY person who has done a lot hands on and read a lot, but I’m not a plumber so I still have blind-spots.