MacBook Pro 13 inch sleeps unexpectedly

I have a macbook pro 13 inch mid 2012 i7 processor, 8 gb RAM and 750 gb HDD""'. Issue am facing is that it goes to sleep unexpectedly and randomly. even if it is in between startup procedure and there after it won't wake easily at times it takes 30min to an hour or 2hours time to boot up again or it might wake up instantly. I visited to an apple service centre they advised me to reinstall a fresh copy of the OS which I did the issue still persist. I have tried SMC RESET NVRAM RESET''' also but nothing worked for me. I would really appreciate if you could guide to resolve the issue at the earliest possible.