Can't get filter tank cover off..

Hello. We moved into this house fall of 2015. So last summer,
we used the pool for the first time. It's an above ground pool about 10 years old. Pump and filter also ten years old. Halfway through the summer I had to replace the pump. Filter still working good. Large Waterway cartridge filter. Ever since I closed the pool last fall, I can not get the filter tank open. I think whenmy uncle came to help me with something he really over tightened it. But nothing is getting it open. I want to clean the cartridge to start the season off right! Any tips or help is appreciated. I can take a picture of the tank if that helps! I hope to open the pool this week! 😁
Dawn