That issue can occur when a large MySQL upgrade takes place (such as when upgrading ones distro, or possibly when manually upgrading to a third party version of MySQL).
You can use the “mysql_fix_privilege_tables” tool to resolve that.
You can see an example of the syntax in this thread here:
Some MySQL versions don’t have that program, and instead have a tool named “mysql_upgrade”. You can see an example of the syntax to use for that in this thread: