Usually the rewind spring or spring anchor points are broken. The starter must be disassembled and the parts inspected. Remember the spring is under tension! [There are many different types of starters, and a manual is a good idea if you are not familiar with them.] If the starter is hard to pull, remove the starter and then try it. If the starter is ok off the engine, check the engine itself for a seized or oil-locked cylinder. Remove the plug and pull rope the motor, if it will now spin and oil blows out the plug hole- then gas from the carb is leaking into the sump; or the engine has been tipped for a long time