An Oracle server that had no maintenance for way too long, would not start the database anymore. Trying to start up the database gave an "ORA-03113: end-of-file on communication channel" error.
Some Googling led me to https://dba.stackexchange.com/questions/49682/ora-03113-end-of-file-on-communication-channel-on-startup
. It described my problem and the solution(s) exactly, so I thought I'd share it here.
The output of the adrci "show alert" command looked like this in my case: