It's possible that prolonged time not upright, possibly made worse by the drop in surrounding air pressure and caused the system to leak, or allowed an air bubble to accumulate within the lever's piston area.

Essentially, you would have a pump that needs priming, so will have from zero to minimal action.

You need to to a complete fill and bleed procedure, and hopefully the user guide will tell you how. On some systems, you use the fill port, while pumping the lever and leaving the brake end where oil can flow into a capture bottle. Pumping against no load allows the pump to self prime.

However, don't do anything until you learn the specific remedy for your brake.