Sorry but what do you mean by 12v lines ?
If you check a wiring diagram of a standard SATA power plug, the yellow wire connects to the final 3 pins of the 15 pin SATA power plug and provides 12 Volts of DC power for the drive motor to make the platters inside spin around. The red wire provides 5 Volts which powers the components on the hard drive's logic board (the circuit board the power connector plugs into), as 5 Volts is not enough to get the drive motor spinning. If that red wire has an issue and can't provide a stable 5 Volts for the logic board on the hard drive to operate properly, the drive can spin up all it wants, but the rest of the computer won't detect it.
As fzabkar has alluded to, the problem could lie elsewhere, but to be sure you don't have an issue with that particular SATA power connector, I would unplug the one you had connected to the drive and try a different one to see if it starts working.
Edit: It's also just proper troubleshooting procedure. If a hard drive doesn't work, and you're narrowing down the problem, you need to test everything to find out where the problem is actually located. Replacing the data cable but not the power cable doesn't really help you figure out what's wrong.