Yosemite will not install on my IMac I5

I am having a problem installing Yosemite on my IMac I5 ( 21 Inch Mid 2011) I am currently running Mavericks 10.9.5. and wanted to upgrade. I have upgraded my Macbook Pro I7 and my IMac I7 successfully. I backed up everything and went to the App store to get Yosemite. It started downloading and I got the Yosemite Downloaded in the App Store (Main OS X Yosemite Screen). I expected the Yosemite installation process to start followed by a restart of my computer and subsequent Yosemite installation completion before proceeding with further upgrades as required (i.e. Camera Raw). Nothing happened I did not see the Yosemite in the download folder (although I did not think I had it in the download folder on the other installs I did). I tried a restart thinking that the Yosemite installation process would kick in but nothing happened.

As it stands right now the App store says that Yosemite has downloaded (I do not know where it is downloaded to) but absolutely nothing else has happened. I am in a quandary of where to proceed next. I cannot even download Yosemite again because the App Store says I have already download it.

Any help would be greatly appreciated.

Thanks Tom Nash