Intellisense in IntelliJ no longer working

Check to see if you accidentally turned on Power Save (File/Power Save Mode) I myself just discovered that File/Power Save Mode does turn it off. I was having the same problem and must have accidentally set Power Save Mode to on.

For me it was because I did not have a folder marked as Source Root (it appears in blue).

To fix, right click your root source directory -> Mark As -> Source Root.

File -> Settings-> Editor-> General -> Code Completion-> Auto pop up code Completion

I'm using Mac and when I upgraded to Big Sur my intellisense stopped working.

This is due Big Sur has a shortcut for Input Source. By disabling Mac shortcut, will make Intellisense work again.

You can see the option to disable in mac: