Help! Shower will not turn off

Our shower has a Grohe faucet (I think it is a mixer valve/it has one handle). I dont know the model but it is about 20 years old. Today when I went to turn off the water after the shower it just spins 360 degrees when I turn it and will not "catch" and turn the shower off.

I have had to cut the water off to the whole house.

Now I am trying to take the handle apart to see what I am dealing with an am stuck.

I attached photos. I was able to remove the screw that goes in the center but this black plastic part of the handle all the way to the blue/red markings just spins as one unit. Should it lift/pull off?

There is a hols in the top (shown in one picture) that I thought might have a screw in it but can't see anything. I have tried an Allen wrench and regular screwdriver.

Am stuck here. Anyone able to guide me what to do next and what the root problem may be?