First things first. Your computer may just be having trouble "finding" the system/hard drive.

Follow these instructions first to see if it's there but just needs a little directing.

How to choose the startup volume
Follow these steps:

Turn on, or restart, your Mac.
Immediately press and hold the Option key. After a few seconds, the Startup Manager should appear. The Startup Manager scans for available available (I just love finding typos on Apples site) volumes.

Use the left and right arrow keys on the keyboard to select the volume you would like to use.
Press the Return key on your keyboard to start up the computer from the volume you selected.