[Mashiro]

You should check the logs for errors. You said you changed nothing but other things on the server change in the background without your interaction. Example: something is creating logs and filling up disk space. Or something is not configured very well and uses up the whole RAM after a certain amount of time.

We need way more information:
- Which OS are you using?
- Which web server are you using?
- Which PHP version are you using?
- Maybe most important: which database server (and version of it) are you using?

Logs can be mostly found in /var/log if you want to take a look. MyBB might also create a error log file in the main directory of your forum (I think it might be called error.log or something like that).