Very seldom will you be able to unseize an engine. There are times though when you get lucky, and work it free and it comes back to life.
This is what I do. Remove the spark plug and add a few drops of oil down the plug hole.
Then remove the recoil assembly. This will give you access to a large nut that is screwed onto the crank of the engine. With a socket and a breaker bar work it back and forth without putting an enormous amount of pressure on it. (this is a case of slow and steady is much better then brute force) If it is going to come free this should do it.