Hi Lisa, I'm glad your axolotl recovered without intervention - it's horribly distressing to see a cloacal prolapse, and it's great that the condition appears to have righted itself without anything drastic happening. As Kim says, keep an eye on her, but remember that it's possible that it can be rectified if it's persistent.
We were given some advice after the event as to possible causes of the prolapse. Peely (our axolotl with the problem) appeared on x-ray to have swallowed a lot of sand, and still retained a few pebbles from the brief time she was kept with mouth-sized pebbles. She had also just lain eggs, and produced a fairly large lump of faeces. These, and a possible calcium deficiency in her diet (also picked up on x-ray) were thought to be contributing factors that all came together.
Just to summarise what happened with Peely - we noticed that she had a prolapse on the Friday night after passing faeces, and came on here in a bit of a panic on the chatroom. The next day, we found a vet who would agree to take a look at her, and in consultation with the Small Animal hospital in Edinburgh, attempted to replace the cloaca with a cotton bud. This was a much more complicated procedure than it sounds, as axolotl and cloaca are both quite slippy, axolotl skin is quite friable. Even with three vets and two concerned owners helping, the cloaca could not be replaced.
We were referred directly to the Small Animal hospital, where Peely was seen by their exotics vet (nice guy, BTW) and over a week, they replaced her cloaca a couple of times, and kept observing her to make sure it didn't pop out. We brought her home, and she relapsed again. Back to the Vet School we went, and they tried replacing the cloaca again (also attempting to bind the sand with orally-administered compounds). This time, it held. Peely's now doing fine, but required a lot of attention immediately after the treatment - myxazin and salt baths, application of topical (there's a neat trick for avoiding skin damage while doing that involving wet incontinence pads) and oral antifungals.
Many apologies for the late arrival, I wish I could have been more reassuring a bit earlier.