Hi, I tried to upgrade my Mac Book Pro OS 10.6.8 to Yosemite via the App Store. Halfway through the download there was a power failure which interrupted the download and now some of the programs (like Mail) wont work anymore. Also, when I try to initiate a new download of Yosemite the download wont start. Is there something like "System Restore" on Windows to allow me to go back to where I was prior to the upgrade attempt? I would really appreciate advice on this....