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.