ryobi 40cc 16inch chainsaw?

Take off the starter pull cover and the pull mechanism (4 screws). Does the pull mechanism run freely once off the saw? If so then you have a problem further in. This may include a seized engine which is something beyond the scope of this answer.
If the starter pull itself is jammed, then examination should enable you to work out how the thing is meant to work and correct it.
To check for a seized engine, once the pull mechanism is removed, try to turn the engine crank by applying a spanner or socket wrench to the nut on the end of the crank in the centre of the fan. The motor should turn fairly easily and you should feel compression building as you turn - like turning against a spring.
An exploded diagram is available at http://www.ryobispares.co.uk/assets/applets/PCN4040.PDF to see what parts you may require. If you need new parts however, be prepared to pay a lot more than the price quoted on the parts list.

Nanook of the North
January 2011