This could be because of a few reasons:
You tried to start your server at a peak time.
You installed an outdated plugin. You can check to see if it is this issue via your server logs. (Remove the plugin and then try again)
There are too many operations going on in your server, eg if you have too many skripts.
If this still doesn't work, try to repair your files from the 'Danger Zone' section of your panel.