Check the VTA - vertical tracking angle. I had a very similar, very frustrating experience recently, and taught myself to adjust the height of the base of the tonearm. I raised it up and then slowly lowered it down in increments until it hit the spot where the sound was clear and beautiful again.
Sudden problem with turntable
I have a rega p5 with a ortofon black. it used to sound great. yesterday i put on a record and it sounded terrible, really muddy. My knowledge of turntables is very limited. Is it possible that i damaged the stylus? Any suggestions as to how i would determine what happened?