I am having trouble start my macbook pro. When I hit the start button the chime sounds and the grey apple and rotating wheel appear. After that nothing but the apple and wheel. Desktop does not open. power source is OK. Tried SMC reset but nogo. Help

You need to reinstall OS X. However, you have not told us what version of OS X is currently installed, so it's not possible to inform you further.

Try pressing the power button and holding Command R until the computer starts up in Recovery mode. Run Disk Utility and repair disk or permissions on your hard drive. If this doesn't work then you can reinstall the OS from here as well.

Good luck and I hope you have a backup!

You'd also better hope the computer is running Lion or later, because your instructions don't work for Snow Leopard or earlier.

So to be helpful, as far as I recall with Snow Leopard and earlier you put the OS install disk in the drive on start-up and hold down Command C which should allow you to boot from the install disk. Disk Utility is available in a pull down menu.

Hope you have the install disk in this case!