MacBook Pro stuck at white screen with apple logo and loading bar
A white screen is indicative of either failing hard drive, corrupt system folder or a failing hard drive/IR cable issue. There are know issues, so please tell us your exact machine and what OS X you are running. Also if you have recently updated or upgraded your system, have virus protection or updated any software or any accident happened to it.
Replace your hard drive/IR cable with this one: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable
On some models the hard drive retaining bracket has to be modded to accommodate the wider cable.
I was get it too, after instaling high sierra from elcapitan on my macbook pro early 2011 with HDD. After download and instaling success the system require to reboot. After reboot and apple logo loading bar, it stucked in whitescreen. Any shortcut didnt give me good news. I cant access disk utility, internet recovery, recovery volume, usb osx instaler for downgrade and reinstal, etc. High sierra made my mac broken. Hate it !. When i click power + option only show macOs instaler volume.