Hi,
CARBr6 - this post link below will help with dis-assembly, as most SAA Colts have similar internals.
https://mp40modelguns.forumotion.net/t2801-hws-colt-saa-model-gun-kit?highlight=colt+saaThe most likely cause of your fault is the twin armed leaf spring that pushes up the cylinder stop/lock.
You can easily buy or get one made in most countries or have a fried make one out of some spring steel.
The part is highlighted in these diagrams (copied from the post above).
I had this part fail in my long-barreled MGC Colt SAA, and made a replacement for it.
Just be aware that when you strip down the colt, that you do so over a container to catch any springs and bits that will fly out!
Look carefully to ensure you've got everything accounted for before re-assembly!
Of course, the fault may be something else - but that's a good place to start.
Good luck mate!
Kiwigunner.