the chain came off and when i put it back on it won't go around the bar
Chains do come off when they heat up if they are not tight.
There are usually 2 bolts on the side of the bar. Break these loose then there will be a screw to tighten /loosen. Loosen the bar move the bar back until the chain will go on.
Once the chain is on tighten the chain and rotate it by hand pulling down in the middle to check for tension. Once the guide no longer clears the bar but there is a little slack tighten the 2 bolts and you are good to go.