Yesterday I was unable to connect to my local instance of MSSQL Server, 'localhost/sqlexpress'. This morning I am able to connect without any issue. I can't seem to figure how this could happen. The error I received yesterday was more or less the "cannot find instance". I tried several times and made sure the instance was named correctly. I am not sure how I was able to just connect this morning.
Besides the SQL Service being turned off, you may have had the SQL Browser service turned off. If this happens again, try connecting to the instance via ip,port or even name,port rather than name\instancename.
I am assuming you probably rebooted in between. The most likely explanation is that the service had been turned off, and by default it will normally turn back on when you reboot.
Also, you can sometimes just get weird errors that clear themselves up after a reboot on a desktop machine. This should not happen much on a server, but most desktops have programs opened and closed repeatedly and due to memory leaks and other program interaction that can cause unusual events that are very hard to explain fully or recreate but that will go away as soon as you reboot.
