Bitlocker encrypted disks

Recently I've been reading some posts here about bitlocker in various scenarios, and I would like to hear your suggestions in the worst of these cases which is when you get a turned off system with a disk fully encrypted with bitlocker.

what options do I have in such case, how do i proceed?
links to any guides or documentation are appreciated.

bitlocker case cases computerforensics disk documentation encrypted guides links options system