91 rm125 won't start?

I was out riding it for like an hour, ran great,real fast but towards the end it tried to shut off.Then it took off for a second but shut off and now it won't start,it won't even roll start.Does anybody know what it could be and if so is it majorit made a loud backfire when itried to roll it off and someone said it could be something like the flywheel key

My guess would be that you might have either really old gas, or an improper fuel/oil mix, leaning on the rich side I would say. Buy yourself a brand new spark plug, and if your fuel is mixed right, then it should come back to life. Oh yeah, is your air filter OK? Maybe it's clogged up.

for 1 the plug might be knocked out or the bottom end come out meaning your going to have to buy a new rod and it will cost about 200 to 300$

Clean the idle jet inside of the carburetor .Put it back on with a new fuel filter.If it`s got spark I think it will now run.

Make sure that the sheer key, that holds the fly wheel in place has'nt sheered.....also check the timing....you will see 3 lines...use the middle one..