No Bootable Device problem on MacBook Pro

Okay, so I wanted to try to install Windows on my device using Bootcamp assistant. I partitioned it, but did not continue with the installation because I found out about parallels Desktop and changed my mind. After installing windows 7 using parallels desktop I remembered the partition and went to bootcamp again and deleted it and returned my device to one section. My laptop kept working normally until the battery drained. No problem right? Wrong. I plugged it in and restarted and a black screen comes up saying "No Bootable Device" I googled this and figured out the "hold the option key down" trick. This works and i was able to get back in, however i restarted it again and the problem is still there, how can i permanently get rid of the "No Bootable Device" screen?