I will assume that this is a production box.
try connecting from a different PC to eliminate any possible issues with that particular PC
Ping the Server. If unsuccessful reboot
Otherwise try using computer manager (on another computer) to view the event log on the Server. Look for anything unusual.
Again remotely use Perfmon to look at Processor and Process to see if an individual process (hopefully unrelated to SQL is tying up the server and then you may be able to kill that process)
USe SQL Profiler to see if anything can connect to the server
Try using OSQL to connect at the command line and then run sp_who or sp_who2
If none of that works or gives you any clues then try and restart the SQL Service, or reboot.
Principal Consultant and Trainer