Problem with installing new hard drive in Optiplex 780

Hi everyone,

So, the law firm I work for recently bought refurbished Dell Optiplex 780 from NewEgg. It worked fine for a few months, then it failed to book. Diagnostics gave a code (I forget the code) that indicated the problem was a hard drive. So, they bought a new 250GB hard drive that I made sure was compatible with the rest of the hardware. It's just a Western Digital 250GB 7200 SATA hard drive.

I connected the 2 cables (1 goes to the mother board, the other is like a ribbon cable that goes into a box with a jumble of wires inside the chassis).

I installed Windows 7 and even Microsoft Office, mostly without difficulty.

Now, a few days later, when I turn the PC on, sometimes it boots properly, but most of the time, I get the message "No boot device". I've tried messing around in the Setup (pressing F12), changing the boot sequence, the types of drives recognized, etc. Restoring the settings to defaults didn't help either. I also checked and re-checked that the cables were connected securely. Sometimes the drive is one of the options in the boot sequence setup, and sometimes it's not there, like it doesn't even see it.

What am I doing wrong?

Thanks in advance for any help anyone can offer!

-Sean