There are several problems you can have with a pull start. The main problem is that the pull chord breaks. Repairing this is covered in the video for this topic. If you go to pull the chord and it won't come out either the engine is seized or the outboard is in gear. If you pull on the chord and it comes out easily without turning the motor there is a problem with the spring-loaded dogs that come out and connect the pull start mechanism to the flywheel.

Freeing up a seized engine

An engine is said to be seized when it is physically unable to turn. This can be due to corrosion, a bent conrod, bad bearings or a number of other causes.