How to troubleshoot macbook's failing parts Not sure if its a software or hardware problem. When I turn on my macbook pro (2011) it asks me for a password, shows some progress bar after (it fills up to like 30%) and then it just reboots and starts the whole process again.

I tried all kinds of recovery methods (USB stick with repair tools, another USB stick with a fresh version of Yosemite, Internet recovery...) but I always end up at a grey screen and nothing is happening.

p.s. Apple hardware test says that everything is ok and no errors have been found.
Try resetting the SMC and resetting the PRAM.

It's also possible that your hard drive is failing in such a way that the computer can't boot. I haven't seen that often but it can happen (if you hear weird clicking sounds on boot, that increases the likelihood of this being the case).

It's pretty easy to take the hard drive out of your computer (all you need is a a phillips-head #00), it may be worth removing the hard drive, then trying to boot to an external disk.

MacBook Pro take-apart guides.