I believe you can find the gpu's separately on places like amazon but it is pretty hard at the moment. My suggestion in your case would be to contact lenovo or the shop you bought it from if you have any warranty left. Warranty is our best friend in scenarios like this. Also you could try to find the original vbios for the gt750m(easily found on forums, google) and just flash that and see if it works. If it works then you're all set, I doubt it's defective hardware as these have problems sometimes but you never know. Let's hope for the best and do post back if the flashing doesn't work for some reason.