First of all, I'll apologize if someone else has posted the same/similar solution.
Now, You're going to need to pull the outer casing off and detach the entire pull assembly. Be careful not to let the coil spring unwind any further than it has, if not already unwound. Next, you'll need to inspect the assembly to look for a cause for it coming off the spool. Chances are it was just loose, and became dislodged. However, the spring may be broken or something may be worn to the point that it needs to be replaced. Obviously, if there is a problem and you ignore it, it will just come off the spool again and all your trouble will be in vain.