You can run the command "cf status" to see if the other node maybe up and ready for giveback.
If not - the system has a physical COM console port which you can connect to with a standard RJ-45-Console cable.
It also has a service processor (SP) IP based remote-control port (a bit like ILO/iDRAC/BMC on servers). You can find the IP of the working controller SP with the command "SP status" and perhaps guess the non-working controller IP and try to connect to via telnet/ssh (user is "naroot" with the same password as the "root" user on the controller itself).
Once you connected - you can run the command "system console" to jump into the "physical" console port of the controller - and troubleshoot any boot/disks issues.
It still unclear what the status of the external shelf from you reply., if the SAS HBA on the second controller will still not recognize it some pictures of the cables/LEDs and physical troubleshooting will be needed. (can my maybe do some rests/power-cycle via ACP if cabled - but it seems that it's not or the shelf down in the output which says "System ACP Connectivity: Partial Connectivity")
s0u1sanb(takeover)> cf status
s0u1sanb has taken over s0u1sana.
So, I can connect only on SP prompt :
Which SP commands should I use to correct Theses issues : - Environmental failure on shelves on this channel? yes - Interconnect Port: port not active - System Storage Configuration: Single-Path HA
- System ACP Connectivity: Partial Connectivity - slot 0: SAS Host Adapter 0b (PMC-Sierra PM8001 rev. C, SAS, <OFFLINE (hard)>)