The simple problem would be just that the lockout mechanism needs bleeding as mentioned before it could also could be hydrolocked. My moneys on the lock out though, push it all the way in and hold using the most of the pressure in the system and push down on the fork if the fork opens up it just needs a bleed.
If you had the moco cartridge out you would see how the system works when the x-loc is pushed down it pushs a rod down which opens the gate on the loc out mechanism. Without any pressure on this gate the gate stays closed hence why a bleed may fix the issue.