If it is a gas rig, make sure the ignition is working. I recommend picking up both a spare set of spark plugs, and then one of those cheap spark checking devices and make sure you are firing a spark to the plugs. If the spark plugs are working, and the choke was open while you sprayed in the starting fluid, it will start - guaranteed.
The most common problem with not starting in cold weather is over choking and flooding the plugs so they will not spark.
Other things to look for is corrosion on the ECM or key switch connections, resulting in no ignition at all. Sometimes, part number 21199, the igniter; or 59026, the pulsing coil can fail or have wet and corroded contacts.
If the rig has been starting hard and taking a long time to warm up, and then conking out once in a while in warmer weather, then I would lean toward the igniter or pulsing coil as the source of your problem. The last two items do not fail that often, but when they do you start having low power, difficulty in keeping them running when cold, and an occasional time when the engine just dies for no reason.
Most often they are damaged when someone jump starts the Mule from a running car or truck. The car or truck alternator overruns the Mule's electrical system and blows the internal IC's in one or both of those parts.