Sounds like a GPU failure, which was a common problem with 2011 MBPs. Check repair link above^. Hopefully that covers you.
@havSerial @bikes_knob Serial No says it's a early 2011 model. Shall I take it to an apple store or an authorised repair center will do? Are both equally likely to do repair for free if the Mac is faulty?
@frankohara started
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Sounds like a GPU failure, which was a common problem with 2011 MBPs. Check repair link above^. Hopefully that covers you.