I have been reading a "lot" of users who's system just did not starts once it has been turned off or after a restar command. Some things in common: Virtual machines, Boot camp, multiple partitions, I exactly do not know what could it be. But, behaviour is something about addressing the startup sequence.
At least in my case, my MBP (mid2009, 8GB, 1TB HDD) progress bar just keep static at 0%. The HDD has zero activity. So, it looks like it receives the command to start, but then do not know exactly what to do.
I will appreciate some help trying to fix it. As you did read, my MBP is pretty old, so battery is an issue. I must keep it fully charged to avoid restart from zero.
Each time I must reinstall and it takes 1 hour from pen drive an a lot of network activity.
Apart: My MBP is now a lot of times faster than using previous OSX. It has this awful pitfall, but in the other hand is a really excellent work.