Since you say that tis worked before, and assuming that nothing has changed (any updates, or other types of changes) you might try a nvpram reset: Power off the MBP, power back up holding the following keys, option, command, P,R (no commas) these keys need to pressed and held before the start chime. Hold these keys till you hear the start chime 2 times then release. Reboot. See if this helps. Included are some links that may or may not help, but worth readinga s they may give you a "brain tickler", as your profile does not have much detail, I am including some stuff that may not apply.
Hope this helps.