Are you able to connect a serial connection, direct connection to the appliance? Confirmed that the upgrade completed, and configuration loaded? View this from status on the CLI. Also - if it is a C150 - run version and verify that there is not a RAID issue, or a RAID rebuild running that may be consuming resources. If the appliance does have a failed drive post reboot, which sometimes does occur with the older appliances... if you are still under contract, open a support case and we can assist to troubleshoot, and possibly RMA - if you qualify.
The C150 will only upgrade to 7.6.3-019 - so, that will be the latest revision we have that is supportable on that appliance version.
Hope this helps!
-Robert