You are in multi-window mode. You closed the Tool Options. You closed (or maybe minimised) the Layers / resources dock. Why did you do that?
There once was a serial breaker of Gimp 2.8 on the old gimpforums.com Some videos I made to help.
https://youtu.be/i2sKkA3vqvw reset gimp 4 minutes Which you have already done but more information in the video.
https://youtu.be/vqEUZvvhFKY restoring 2 minutes more dragging docks around
https://youtu.be/lkAPp1ckCNs 6 minutes further setting up Gimp 2.8 interface.
Have a look through those. Back to my original advice, reset Gimp to defaults.
Then
Do not move the Windows around. Put Gimp into single Window mode