NX650'89 sudden stop and lost compression

[RuiNx650]

Hi!
Yesterday, I went to my mechanic to solve a fuel consumption problem, that revealed a stall valve! I went back from the shop, made 40Km's, and it gives me a big smile on my face! Today when I came to work the engine sudden stop and I've notice that It as almost no compression.
Can anyone tell what I'm about to face ?

Hint : A few time ago my mechanic told me that the arm rocket(all) and the cam shaft have a high ware.

[Lumiya]

Loss of compression is one of three things valve open (due to stuck vavle/cam/chain, Spark plug popped out (due to being loose or stripped thread), Piston problem (broken ring or holed)

[IgorVigor]

or the head gasket has gone....

[Rubberchicken]

Or a valve seat.

[DrSmith]

In any case, that head has got to come off.

[catteeclan]

A good way to find where the comp's going is the fire air down the spark plug hole using a special fitting. At least you'll know what you're facing before the strip down.