First thing I would do is remove the side plate and see what is going on inside the action. Go from there, carefully. Need I mention to have the FP over the fired cartridge.

...thankfully, it was not loaded at the time. It's at Alan Harton's place.

There could be a number of reasons why you can't you can't cock the hammer and rotate the cylinder. I had had this happen a number of times. Most often it is a ammo issue. Check and make certain that a bullet has not jumped the crimp and is tying things up. If it is, work it back into the case...gently.

If it is just dirt, grit, extruded primers etc, hold the latch back and tap on the side of the cylinder with a rawhide mallet or a chunk of wood. Nothing steel or even brass thank you.

I am a bit concerned when you said, you can't hold the cylinder latch back. These old Colts have a fairly complex lockwork and sometimes if the side plate is not down tight, things can jump out of place. If you can't get the cylinder to swing open and the latch won't come back all of the way, it is time to remove the side plate and start looking at and removing internals. The handgun has a V mainspring and one leg powers the hand and the other the hammer. The leg of the spring fits into a narrow shelf in the side of the hand and if the side plate is loose, it can jump out. It is easy to nudge the leg of the spring up and back into the recess in the hand. Much past that things get a little more complex. If you don't know your way around this lockwork, best let somebody that does do the work.

Don't start beating and prying on things and create more problems that you have now.

I was pretty sure it wasn't an ammo issue as, cocking the hammer partially back, allowed the cylinder to turn freely. So, I grabbed a handy leather thong and blocked the firing pin, letting the main spring hold it in place and popped off the side cover. I was able to see that (here we go exposing our ignorance on proper part names again) the hammer block(?) was down preventing the hammer from being drawn back and also keeping the latch from opening. Slid that out of the way and was able to open the cylinder and unload. MAJOR sigh of relief.

One thing noted...my habit in choosing the correct bit for any screw is to take the bit in my fingers and try it for fit before mounting it to the handle. Doing so, revealed the side plate screws were less than finger tight. "Hmmm....", sez I.

The hammer block connects through, as Charles notes, "a complex linkage" to a pin on the right side of the trigger. The linkage itself it thin...perhaps .010" or so thick. A bit of juggling got the linkage reconnected to the pin and all seems well.

My theory is the side plate was loose enough to allow the pin to 'jump ship' vis a vis the linkage. Reassembled and everything works like it did. Time will tell if it was a case of loose screws or wear over the past 98 years. At least I have a bit of an understanding what goes on inside this one.

Thanks to all for the advice!