When my computer restarted unexpectedly, I was unable to open the Intellij IDEA IDE due to a Cannot connect to already running IDE instance. IllegalStateException: Process "/app/extra/IDEA-U/jbr/bin/java" (2) is still running. If the process is an instance of the IDE, please collect the logs and contact support. If it isn't, please delete the stale "some-path/com.jetbrains.IntelliJ-IDEA-Ultimate/config/JetBrains/IntelliJIdea2024.1/.lock" file and restart the IDE..
Basically, the app thought there is a running instance due to the unexpected reboot. All I had to do was follow the exact steps above. I went to some-path/com/jetbrains.IntelliJ-IDEA-Ultimate/config/Jetbrains/IntelliJIdea2024.1/ and deleted the .lock file.
In the case where you do not know where that .lock file is, you can run the command find ~/ -type f -name '.lock' to find all existing .lock files. Be careful not to delete the instance of an app that actually is open!