PC suddenly low FPS?

Hi, my brother's PC was assembled by me & ran perfectly for 3 months now. No problems and more than enough FPS.

gtx 2070

ryzen 3600x

32GB RAM

etc.

The games have been going this morning and extremely bad. Fortnite otherwise at over 180 and now partially below 20 and does not exceed 30 FPS. The same with other games like Cs Go and apex legends. There's still a picture. The computer does not make any unusual noises. Does anyone have an idea why this might be?

All drivers installed?

is the graphics card also selected? Sounds more like using an onboard graphics card.

is a cooler broken? Does the computer overheat? That sounds like an emergency operation