There are several differnet things with several different causes
One problem. I often ask people if they know what the biggest problem is with a Class A? They shake their head, I hold up most of my key ring (The ignition key may be missing) and say "They all fit" (on my ring the Ignition key is detachable Just three keys on the "Sub" ring, Ignition (MH) Ignition (Car) Bicycle padlock.
All that weight can cause damage to the ignition switch.. at least on SOME vehicles where the cylinder (lock cylinder) Operates the switch dirtectly.
ON other vehicles the actual swiche may be anywhere from an inch to a couple feet away operated by a rack (the cylinder operates the mathing Pinion) and again all that weight can cause failure of the rack to fully operate the switch.
Another failure mode... You turn the key and it acts like dead battery...
Low battery, Old battery, Dirty connections (Mine used to be the batteyr positive till I fixed it) BAD STARTER.
Older vehicles with cam shaft operated fuel pumps (Gasoline type) Suffered Vapor lock. Theory has it the newer "in tank" fuel pumps are Vapor Lock proof.
And this is just a short sample of different "It will not start" and different causes.