New hard drive in early unibody 2011 MacBook Pro won't boot
Reboot the computer and hold down the option key so you can access startup manager. 

If you've already installed OS X, select your HD from the system.

If you NEED to install OS X, select your USB key and run the install

Another option would be to restart the system and hold down Command + R to enter recovery mode. Depending on your Mac model, you'll actually be able to install OS X from the internet. It will be the version you shipped with, but you can just follow the upgrade path to Yosemite.

And if you have to use your computer today, I'd just throw in the old hard drive and deal with this over the weekend