SQL Server inexplicably stopped working
Based on the comment thread, the first step is to update the system. That is, install Windows updates, patch the Sql Server and update all the device drivers and firmwares. On virtual machines (which you seem to have), the VM guest drivers and the host itself should be updated, but that's usually not DBA's domain.

Lack of patching is often a red flag about more widespread infrastructure problems. Please make sure that the system is fine in other ways. That is, you have a working restore plan [1], service accounts, passwords and user rights are documented and so on.

To illustrate the patching effect, SentryOne has a handy list about the number of fixes included in service packs and cumulative updates. Just SP2 fixes some 133 issues, of which "only" 53 are public ones. Reading through all the release notes about if the problem is mentioned is going to take a while.

As a side note: Microsoft's support policy is restrictive. One needs quite strong a business case (read: lots of money is at stake) if unpatched software problems are looked into. The advice usually is akin to "install all the patches and see if problem occurs again".

[1]: "Working" means you have tested recently enough that the backups can be restored and contain the stuff they should. It's all too common that backup process looks successful, but doesn't quite do what was intended.