Q: My MacBook on startup shows file with question mark ,, what now?

Press the power button down to force a emergency use only hardware shutdown.

Press and hold the Option/alt key on the built in keyboard, boot the machine.

If MacintoshHD appears, select it and click the arrow, then in System Preferences > Startup Disk reset that. Done.

Sometimes a NVRAM reset is required then the above done again.

Folder with question mark issue

..Step by Step to fix your Mac

If MacintoshHD doesn't appear, or if it boots to gray screen or other issues, then it's more complicated of a fix, but once inside OS X then reset the Startup Disk.

Gray, Blue or White screen at boot, w/spinner/progress bar

..Step by Step to fix your Mac

You might need this if you don't have a recent backup and your problem is software related, Disk Utiltiy can't fix the drive and recommends you backup, erase and install.

.Create a data recovery/undelete external boot drive

If Disk Utility + Hardware Test shows no boot drive, then you have a dead drive or cable, or Mac issue.

My computer is not working, is my personal data lost?