on Monday i came in to find people screaming as they couldn't log on. had a look at a photocopier - saw it couldn't authenticate so went to look for servers.... all were on and working normally, so looked at switches. ahh found the problem. our old core switch (3com 4950) had all it's packet light's on, which it has done before. so i unplugged it and all the network/fibre's, waited about 10 seconds and pluged it back in, then the fibers one by one and then the copper network. it didn't lock up, great problem solved i think - goes to photocopier and i can now log in. within 5 mins it had done the same again. I've now plugged all the coppers into other switches and fibers in where i could, and all the network that is connected it running fine. what could have caused this to happen - it had been plugged in for about 6 months with no changes and no issues. The switch in question has done this before but a quick reboot has solved it previously.