I am installing OS X El Capitan on a Mid 2009 MacBook Pro. Previously running an OS X 10.8.5. Incidentally my installation is taking now two days. I have tried holding down the power button but still no difference. It resumes where it stopped I feel frustrated and I want to return to my previous operation system. What can I do? Does it mean my machine has crashed? have I lost or can I recover my work? Making matters worse I can't tell what percentage of installation achieved? Apart from the bar that has moved from 0% up to around 75% and it's stagnant at that level. I am confused on what to do.