I don't know wether the trains have individual E-Stops or not. Originally the system was designed to run without the need of a driver wasn't it? It is possible it has one at the front as a precaution anyway though.
Each compartment has a passenger alarm which can be activated in an emergency under the red flaps though doesn't it? Could be someone pratting around who pressed it and caused an E-Stop.
The odds of an actual crash are very slim. Firstly, the system knows where every train is and what it's doing in normal operation, and ensures there is a safe distance between them all (Similar to Duel). If one train had stopped midcourse due to a breakdown the computer would more than likely be aware of this, and prevent other trains from attempting to pass it.
Failing that all onboard staff usually have radios which can be used to message back to the control room, so if one had halted at random odds are they would radio back to say they appeared to have broken down, and to keep other trains well away.
I have heard many staff from various parks all say that in most cases it is safer to ride stop than to e-stop, due to things such as abrupt halting, and overstraining hardware. It is possible that the train was performing an e-stop for whatever reason, and this owed to the abrupt nature of it coming to a halt.
It could also be, as suggested earlier, debris on the line which became wedged in the train's drive system and caused it to jam, though I would have hoped the onboard staff spotted this and either stopped the train, or radioed control to request an e-stop.
Do we have any idea where abouts the problem occurred? That might help whittle down the possibilities. If it was say, over the coach park the chances of debris are significantly less than say passing between the Valley and Katanga.
With regards Von Roll still operating, the company packed in as we knew it a few years ago now, however there is still a Von Roll Holding operating who make wind turbines amongst other stuff
Monorails and cable cars are long gone though it seems. I believe they sold the Monorail technology to Bombardier or similar (Who incidentally, apparently did some trains for Disney World's network)
Not so sure about C.W.A and Allot & Lomax. Like with most rides where the company has ceased operation but attractions still exist I should imagine Towers can get compatible parts, one ones custom made.
A bit back now it had some parts changed. The system used to use Analogue convertors, but they became unreliable with age, and were changed for digital drives by a company called ABB. This was done to make it more reliable and improve the performance of the trains. I'm unsure as to wether anything was done during the refurb though. As far as I know Sarner and Alton just gave the trains a makeover visually. It is possible it had some parts changed under the hood though!
Odds are it's a fault confined to a single train, as it seems very strange that people are claiming just the one has failed. I'd imagine there'll be a H&S inspection carried out on the whole system to rule out anything else, which'd explain the prolonged downtime and they'll probably then reopen with the defective train in the depot for repairs.
Assuming it's a one off with a single train the worst I can see happening is that train is decommissioned and scrapped/gutted for spare parts. There's always other trains to support it, so I doubt it'd have too much of an impact loosing one