While upgrading my Supermicro server's IPMI firmware, I had Internet drop on me. It wasn't a long drop but it was long enough to trigger dreadful "Service is not available during upgrade" error. No matter what I tried, the same error popped out.
Fortunately, if you're running Linux and have your IPMI tools available, there is a solution. Just cold-boot BMC (small computer within your computer actually providing all those IPMI services) and wait until it's back up:
ipmitool -I lanplus -H 192.168.0.1 -U ADMIN -P ADMIN bmc reset cold
Once BMC reboots, it will forget all about interrupted firmware upgrade and allow you to continue on your merry way.
PS: If you are not sure if firmware update started before connection was interrupted, give it 10 minutes before trying this. This will be enough time for it to finish any real upgrade that might be in progress. You never want to interrupt firmware flashing. And do try in a new browser session - sometime cookies make upgraded firmware wonky.
PPS: If you with to reset the unit to factory defaults, you can try the following:
ipmitool -I lanplus -H 192.168.0.1 -U ADMIN -P ADMIN raw 0x3c 0x40