That depends on if controller B is halted or in takeover mode, you can check this with `cf status` on controller A CLI. If it is in takeover mode then issue `cf giveback` from controller A CLI and it'll power up.
If it's halted you'll need to make a console connection (if you haven't configured the BMC) and you should see the loader prompt. Type `bye` at the loader prompt and the controller should boot unless there are any issues, in which case you'll need to halt boot with ctrl/c when prompted for Special Boot Menu and select option 5, Maintenance Mode Boot.
