You can also run the following:
EXEC sp_helpdb 'TempDB'
And check that the path that it shows the tempdb_log file is at is there and the file is there. If the path is valid but somebody messed with the file restart SQL, it will recreate TempDB.
BTW, if the log file for TempDB is missing, I would imagine that you would be seeng a LOT of errors in your SQL Error Log and Application Log.