27" mid 2011 imac after apple logo stuck whitescreen

hi,

i have 27" imac mid 2011. 3.1 ghz i5 4gb ddr3 and ati hd6970m 1gb

it stuck on white screen

after i push power button apple logo comes and start to loading. after finish the loading. monitor turns to white. desktop screen dont came.

when it turned the white ,keyboard stop to working.

i try to boot windows and press option key after power button. boot menu come i saw boot menu after select windows.

windows 8.1 began o boot after loading when desktop screen must came but still waiting blackscreen desktop wont came.

and while i open with optin key to see bootle hdd. some parts of screen is pink😮

i tried to every thing. unplug all usb stocks and power cable wait 1 min after that press power button upto 20 sec after that plug power cable again but same issue.

tried comand+option+p+r to reset but same issue again.

tried to boot safe mod pressing shhift key but same issue,

last i tried to boot -s mode and got some errors about usb

pls help me whats the problem of my imac.

what should i do to fix this issue.