Had a weird one yesterday. Main fibre went down at one of our sites. But the backup router (On ADSL) didn't kick in. So went to site.
Found the router was plugged in but the port on the HP switch was not showing lights. Checked the config gui and saw the port disabled. But wouldn't let me re-enable. So changed the port and the lights flashed and then off, again checked the gui and that port is now disabled??
So as a test plugged my laptop and router into a separate switch and i could ping the router fine. plugged a patch between this switch and the main one and boom no connection to the main switch??