It's always worth trying to connect via the serial console before rebooting. It might be something simple and fixable.
If the network was down, you'd still get in through the backdoor.
Might just have been very busy too. Given time it might have come back.
A quick reboot is sometimes less hassle though.
You might consider installing 'monit' which can be programmed (easily) to alert by email (and do other stuff) if the load goes up, or you run out of disk space, or a service stops running etc.