-since 4 months My Y50 laptop shutted down suddenly
-I tried everything but MB (motherboard) is dead ,then i lost my 1250$ laptop after 1 year only Mysteriously (OMG)
Because the laptop has one yr warranty only so that you should contact lenovo when you get any problem.
someone offered me to buy me A new MB for about 200$
-I dont know from where he brought it Or how but i really needed it very very fast
That is the problem, because like you said nobody will know where he bought from. Usually the gtx860m is option, that means you have to remove it from the old MB, then put it to the new MB. If the new MB does not have the gtx860m, you can use the intel iGPU only. Now if you got the problem you have to go back to that guy, if he can't solve the problem, you will loss other $200. Sorry.