Upgrade from Mac OS 10.6.8 doesnt work I have a MacBook Pro on OS 10.6.8 I have tried to upgrade to the current OS. The download appears to work fine, but when I select restart to install, the Mac starts to shut down, then just hangs.

I can only start the Mac again by holding down the on/off switch to turn off, and then turning on again.

What is the solution?
Do you have the required hardware for the system you want to run? Here is a list of requirements, I shamelessly copied these from support.apple.com
OS X 10.6 Snow Leopard system requirements
To install Snow Leopard for the first time, you must have a Mac with:

An Intel processor
An internal or external DVD drive, or DVD or CD Sharing
At least 1 GB of RAM (additional RAM is recommended)
A built-in display or a display connected to an Apple-supplied video card supported by your computer
At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools

OS X 10.7 Lion system requirements
To use Lion, make sure your computer has the following:

An Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
Mac OS X v10.6.6 or later to install via the Mac App Store (v10.6.8 recommended)
7 GB of available disk space
2 GB of RAM

OS X 10.8 Mountin Lion system requirements
To install Mountain Lion, you need one of these Macs:

iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)

Your Mac needs:

OS X v10.6.8 or OS X Lion already installed
2 GB or more of memory
8 GB or more of available space

OS X 10.9 Mavericks system requirements
To install Mavericks, you need one of these Macs:

iMac (Mid-2007 or later)
MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
MacBook Pro (13-inch, Mid-2009 or later),
MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)

Your Mac also needs:

OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
2 GB or more of memory
8 GB or more of available space

OS X 10.10 Yosemite system requirements
Before you install Yosemite, make sure you have one of these Macs:

iMac (Mid-2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)

Your Mac also needs:

OS X Mavericks, Mountain Lion, Lion, or Snow Leopard v10.6.8, already installed
2 GB or more of memory
8 GB or more of available disk space