This could be an indication that your computer's hard drive is not set as the first boot device in the BIOS.
Have you recently made any BIOS changes?
If not, are you familiar with the process to make changes in the BIOS? It's a simple enough procedure, but can be a little daunting, the first time you do it. If you need help, post back here, and I (or someone else) will provide step by step instructions.