If tempdb is full (or the disk it is on), SQL Server will more than likely stop working. When you restart SQL Server tempdb is dropped and recreated and will be empty and the system will work again. The data files will most likely still be large though, this can be fixed by altering the data files of tempdb with the ALTER DATABASE command. I suggest that you monitor growth and find out why tempdb is getting full and consider taking steps to avoid this. This could be a redesign of the code that is making use of tempd, or changing isolation levels, or extending the size of the drive tempdb is on or a many number of other possibilities. The root cause needs to be identified to then take the correct course of action.