It's a recently refurbished MacBook Pro 15" from late 2011 I tried resetting the pram with shift+commmand+p+r while powering on. Same prohibitory sign. I tried command+r. Same. I tried command+shift+r for the online recovery. Nothing. I tried holding down shift while powering on to run safe mode. Same sign. I tried holding option while powering on. I didn't have the recovery partition because I had recently swapped out my original SSD for a 1TB HDD. I later found out this HDD had no recovery partition. And sometimes I don't even get the sign, just a black screen (as if in sleep mode). I took out the HDD and opened it from another iMac by USB, reformatted it, erased everyrhing and then replaced it to my MacBook. Repeated the above commands and nothing. I then opened the HDD on the other iMac, and installed OS X El Capitan. It opened and everything. We set it up. We put the hard drive in the MacBook and after turning it off and on 2 or 3 times it booted! For the first time in five hours! Asked if I wanted to reopen windows since the computer restarted for an error. I pressed cancel. And it shut off. Tried the aforementioned commands, same negative result. The internet recovery begins to work for a half a second but then cannot connect. I then try installing OS X Tiger from a DVD, the only OS we have on a disk. This does not work. Then we replace the HDD harddrive entirey with the original SSD one. We open it on the iMac, download and update El Capitan. We place it into the MacBook and the apple logo stays longer. It boots halfway sometimes but then we get black and white letters about kernel panic. This happened with the other hard drive too after installing OS from the iMac. We also try the recovery partition which is on this SSD hard drive but that doesn't work. The boot seems more promising but still doesn't completely work. It's yet to start up again. In a final attempt, we connect another hard drive that serves as a backup for the iMac by USB to the MacBook. Holding option while powering on, we manage to run it and that one does not boot. Same prohibitory sign from an entirely different functioning drive. We try again with ithe recovery partiton of that backup hard drive. That one doesn't boot either. At first I wasn't frustrated but literally NOTHING works. What do I do next? I've tried two hard drives already. What's wrong with it?