Chances are your configuration move from 11 to 12 has a few lines of code in the BIGIP configuration the new version doesn't like. It is stuck in a reboot loop.
If you have terminal access run the following
tmsh load /sys config
It will show you an error like such ...
0107146f:3: Self-device state mirroring address cannot reference the non-existent Self IP (101.10.xx.xx); Create it in the /Common folder first.
Unexpected Error: Loading configuration process failed
If this is similar to the error do the following
cd /config
vi bigip_base.conf
once in vi press '/'
perform a search on the IP listed as the self ip that causes error
comment out the IP address (two sections in this case)
:X to save out of VI
tmsh load /sys config
Should load successfully once all invalid IP addresses are removed, in the event it does not it will present another IP address in error, in this event comment out the other IP address and reload the configuration once again
once the configuration is loaded run the following
shutdown -r now
This will reboot into the volume.