MacBook Pro will not open or start.

Following an Apple Update on 12 July I left Macbook Pro overnight to install same. Next day it would not open /start. Prior to this it had sent 2 reports to Apple because it would not open, but I overcame these by trying again, then again, following the messages on the screen. The next time it would not open/start and no messages appeared on the screen so I phoned Apple Support and was taken through several key-pressing manoeuvres to no avail. I took it to the AppleStore Genius but the only way she could help was by deleting everything and setting up the MacBook Pro for re-installing at home from my back-up drive. This worked. I returned to AppleStore as the 'wheel of death' had been appearing and we transferred several large files from the Macbook Pro to another exterior drive to give Macbook Pro plenty of 'wriggle room'. Apple agreed there was plenty of room on my hard drive now.

On August 18th the Macbook Pro battery ran out. I charged it up but it would not open/start. I returned to the Genius Bar but the Genius could not find the problem. He did the same as the previous Genius - deleted everything from Macbook Pro and I re-installed at home from my back-up drive. Again this worked. The genius suggested I might need a new hard drive but I don't think so as it is working perfectly now and I cannot see how the hard drive is responsible for opening/starting Macbook Pro.

However, I am currently afraid of letting the battery run out as it seems it will mean going through the whole procedure of deleting and re-installing.