This indicates that you have altered or removed the contents of the "mysql" database of your MySQL process in a way that will prevent your process restarting, or render it unusable after restarting. MySQL processes don't restart very often, so it could be weeks or months before the effects of these actions surface, by which time you will probably have forgotten all about them.
If you catch the problem before your process restarts, you should still be able to connect to it and make a backup of your data.
If you don't catch the problem in time, there is essentially no way to repair it; your MySQL process either won't start anymore or will start but won't be accessible. You'll end up having to delete the process and start over with a new one. If you contact us before you delete the old MySQL process, we may be able to recover some or all of your data and restore it to the new one, but if this is possible and you wish it done, a non-trivial service fee will apply to cover MySQL administrator time.
Keep in mind that while our system does check periodically for this type of damage, it doesn't detect all cases and may not pick it up right away. To keep your MySQL process safe and happy, make sure you always leave the "mysql" database alone.
The "FLUSH PRIVILEGES" SQL command will also cause a running process to immediately fail if it has this type of damage.