2009 kx250f

I have a kx250f . I took it for one ride and it just bogged out and shut off won't start . I sent it to a back yard guy he shimed the valves but couldn't close the cap or cap wouldn't open something like that.  The bike won't start has compression . I was also tod cylinder head is gone and needs new valves 800-1000 bucks . Is this true ? or is something stupid going on that we are not seeing here? . The bike always worked good and well maintained but I still have yet to get a final answer on what is wrong . Any help would be grateful.