SATA mode-AHCI or IDE? no bootable device?

Aspire One D150-1165. Used 8-9 times in 2 years.

Turned it on and it says....check cable connection...then says no bootable device. I have searched and found the same problems, but none of the answers helped me. F10 did not work. I know when I bought it that I had Best Buy put more Ram in it. I took it to them yesterday and they said the hard drive had failed. They said I would need the Acer disks so I brought it home. I searched this forum for 2 hours, but none of the answers worked. I had no idea what I was doing, but I began doing what others said to try.

Here is what I did:
1. F10 did not work. F2 gets me into Bios.
2. Bios- HDD shows Hitachi with a number. Is this supposed to have a check mark or anythiing? It just shows as #1. my external dvd Drive shows as #5 or 6. network shows as #4. Does this mean the hard drive is working?
3. Bios- Main-quick boot, network boot, F12 boot and D2D recovery are all enabled.

No one told me to do this next part, so I am now worrying about what I have done. Please tell me if I have done right or wrong....
4. Bios- Main- SATA mode- set as AHCI. I changed it to IDE. I had no idea what it is or what it would do.
5. Saved with F10 and enter.
6. computer restarted and it was back to normal. What happened? Did I do the right thing by changing the SATA mode to IDE?
7. What exactly is SATA mode AHCI or IDE?
8. So since the computer is working...does that mean my hard drive is okay?

Thanks for any help. Please let me know if I have really screwed up or not.
Peke