The most important thing to do in the event of a server crash is to back up your data. This means making sure that you Backup your databases, servers, files, and everything else on your server before anything happens. Then, if something does go wrong, you'll at least have a copy of your data to work with.
If your data is backed up, the next step is usually to restore it. This involves copying the backed-up data onto the server where it crashed so that users can get back up and running as quickly as possible. Sometimes restoring data can be difficult or impossible, depending on the situation. If this is the case, it's always a good idea to contact customer support for help getting your servers back up and running properly.
If all else fails and you find yourself unable to get your servers back up and running, it's probably time to call in a professional computer repair service. They'll know how to quickest fix any problems and help ensure