Chain saw won't start - Poulan PP4218AV 18" Chain Saw

this seems to be a chain brake that's applied , try turning off chain brake by pulling back on hand guard till a click is felt

some folks moving up from an older saw to a more modern saw are not so familiar with chain brakes and don't understand them and how they work

If you are talking chain saw there is a safety clutch lever immediately in front of the top handle. this has to be back against the handle and when the chain jams you simply push it forward with your fingers and the chain stops

Chain adjusted too tight?
Chain Brake loaded with saw dust?

check for spark when it fails to start could be a bad coil

Had this happen to mine when I was limbing stuff that was too small. I know better. There is probably a burr on one or more of the guide portion of the links where they run in the bar. This causes that link to be tight in the bar at this point. The chain probably hit something on the body of the saw when it flew off. Take the bar off the saw and run the chain through the groove in the bar. If you have any links with nicks, they will bind, causing the chain to stick and not run freely. Once you've identified the problem links you can remove the nicks using 120 grit paper backed up with a flat piece of steel so you only remove the high spots.

Check to make sure the blade is not installed backwards. It's an easy mistake to make, everything will work OK, but the saw won't cut if the blade is on backwards.

How many hours on the unit?
Does it have spark?
Is it getting fuel?
Check the compression. If it has low compression it won't start.

Things to check,drain and re fill with fresh fuel/oil mix, replace the fuel filter in the fuel tank, remove the spark plug, rest on top of the cylinder and check for a spark, if this is all ok remove the muffler and check the condition of the piston through the exhaust port, any signs of damage and the saw will have to come apart for further investigation.

Sounds like your chain is to tight.Take the cover that is at right side of the saw motor off and loosen the two nuts a bit and look for a tension screw behind the bar and back it off,if the bar won't move loosen the two nuts that hold the bar on the saw.With the saw siting like you use it pull down on chain and get about 1/8 of an inch between the bar and chain.(use gloves)If the bar is blue by the chain groove You will need a new bar.Blue means the bar has over heated and lost its temper.The blue metal is brittle in other words.You should all so check the drive sprocket for wear.

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sounds like your chain oil-er is NOT working, pull bar and take a stiff feeler gage and go around the whole bar cleaning the groove out and then make sure the oil hole in bar is clean also because if hole going to bar groove is plugged you won't get oil to lube chain. fill with oil and start and run on piece of cardboard or similar to see if it is throwing oil off chain, if so you are good, if not you are looking at cleaning/and or replacing oil pump Good Luck