Doesn't sound too complicated to fix yourself. Take the lock off, see why the sear won't catch the tumbler notches and keep the hammer cocked. Maybe debris, maybe notch sheared off, maybe spring that pushes sear broke. Probably need a new part, easy to install.
based on the description of the gun being Antique. Call Ed Delorge in Houma
DeLorge, Ed www.eddelorge.com/
CUSTOM GUN BUILDER, ENGRAVER OF FIREARMS, GENERAL REPAIR GUNSMITH, GUN ENGRAVER.
6734 W Main St, Houma, LA 70360
(985) 223-0206