It could be that your normal.mxt has gone corrupt. Go to this folder on your c: drive:
C:\Users\<your user name>\AppData\Roaming\ESRI\Desktop10.6\ArcMap\Templates
...and rename the normal.mxt to normal.bkp or something else. Now try restarting Arcmap. The application won't find the normal.mxt so it will make a new one and then finally (hopefully) open up.