[Restrorob]

Hmmm,

This one's right up my alley, Been in the small equipment repair biz close to 20yrs now.

From what you explained "started running rough then quit, Now wont spin over" sounds like a piece of trash got under the carb float needle valve.

This lets too much fuel enter the engine "rough run" and can eventually cause it to die, Once the engine dies the fuel continues to enter the cylinder and will fill it up. Once fuel has entered the cylinder and you try to pull the starter rope the piston moves slightly then causes a hydro lock situation.
You can compress air but not a liquid.....

My suggestion;

Remove the plug and see if this is what happened, When you remove the plug MAKE SURE the plug wire boot is moved AWAY from the spark plug hole and positioned where it can NOT spark to a ground source before pulling the rope.

If this is the fault, Remove the carb float bowl and use carb & choke spray cleaner to clean the bowl and float valve area then re-assemble.

The "Relief" valve that other's have mentioned is actually a "Un-Loader", It's function is to un-load the water pressure on the pump head for easier starts. It also un-loads the head pressure when the wand trigger is released while the engine is running to prevent engine bog.

A bad/stuck un-loader will not cause a engine to lock up but only make it harder to crank over, The oil alert function only kills ignition spark and has nothing to do with the engine spinning over.

Most pressure washer pumps come with a specially formulated oil in them, If one can not locate this special oil a non detergent 30w oil can be substituted. Do not just add this ND 30w if the pump is a little low, Drain all the old pump oil first then re-fill with the ND 30w....

Hope this little tid bit of info helps.