SQL Server will not start.
You open up the ERRORLOG (...which I would only do with a text editor like gVim that creates a cache of the files it opens and can safely read files that have the possibility of being written to while you are reading it...) and you see:
"Could not create tempdb. You may not have enough disk space available. Free additional disk space by deleting other files"
This typically happens after a long day when you should have left for home a few hours ago, btw...