Hi, it sounds like you set a Firmware Password, if you boot holding alt or option key & see a lock in the middle, that's it.
Boot up while holding the Option key down until you see the boot manager, do you see this...
Firmware password protection in Mac OS X ...
It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option boot will show a lock, and Shift, as well as booting from anything but the Hard Drive.
Force Removing Password Protection
1) Add or remove DIMMs to change the total amount of RAM in the computer.
2) Then, the PRAM must be reset 3 times. (Command + Option + P + R).