I have been having an issue with my Mathematica 11.3 installation. Often after running a notebook for a long time, my Mathematica will crash suddenly for strange reasons like just typing in the notebook.
Upon starting up the program, the same simple operations such as typing will cause Mathematica to crash.
The only good solution I have found has been to restart my PC. Even uninstalling and reinstalling Mathematica does not seem to fix this issue.