I have a local in-house client application that relies on steel server MySQL 5.6 database in our closet. This morning, people suddenly couldn't log in. The client server said it couldn't connect with the database. On the database server itself, the MySQL service was running. I stopped and started it again and things went back to normal. However, I don't know what went wrong and I'd like to know why this happened to see if it's something I can prevent in the future. What are some diagnostics I can do to see what happened with the server this morning?