[Ron6519]

My sink has a single handle which controls water flow by lifting up/down or moving left/right. However, recently, whenever I bring it back down, the water won't turn off. Sometimes, I have to turn left or right to hit the perfect spot to turn off. Does anyone know what the problem is and how to fix this? Thanks for the assistance.

You need to identify exactly whose faucet you have. Solutions vary from company to company. As stated before, some have cartridges, some have rubber grommets and springs, etc...
There should be a name on it. Or maybe the paperwork was saved when it was installed.
If not, you would need to disassemble it to see what's there. You could then take the guts to a plumbing supply to get the needed repair parts.
If this is above your skill set, call a plumber.
Ron