I would make a serial connection to the switch. You should be able to access the CLI that way, check the logging and running config, see if anything has changed.
I have a power connect 6224 switch that had everything configured and I was accessing it via the web gui for about a year now. I Upgraded it's firmware a few times before without any problems. today I upgraded from 3.3.7.3 to 3.3.11.2. I did this using the following method: 1. I uploaded the firmware using the web GUI 2. I verified in the versions link on the web GUI it was uploaded 3. I used the CLI to make the image active and update the boot code (boot system image1, update bootcode)  the switch rebooted and technically it's working but i cannot access it using telnet or the web GUI. it seems to have forgotten then login and/or changed the password. What can I do to access it? I have another switch like this and am now afraid to upgrade it as I might loose access to it too.  
Unable to login to switch after firmware upgrade