Yosemite 10.10 Install?
Sounds like a fault, or that perhaps there is a conflict. You are trying to upgrade from 10.7 to 10.10 and its possible you either can't do that, or your machine is incompatible. Although I expect it would tell you as such.

In any case, what ever the reason is your only real way forward is to backup with Time Machine if you haven't already onto an external hard drive, and then do a fresh install rather than an upgrade.

After you've done this during the install process you'll be able to import old backups etc... You can use your backup from Time Machine to restore your files on OS X 10.10.