to commonly asked questions.

File folder and blinking question mark at startup

I just got home and there’s a file with a blinking question mark.

File with a blinking question mark at startup means the computer can not identify a startup disk. This can sometimes be a bad sign. It is possible that the hard drive has failed and the computer cannot find the Operating System. It is also possible that this is a temporary problem and you may be able to repoint the computer to the hard drive. Things to try:

Hold down the power on the computer to completely turn off the machine.
Power up the machine while holding down the x key. This forces the computer to find the OS living on the hard drive.

If that does not work,
Power down the computer.
Restart now holding down the Option key. This will give you possibly two choices. The actual drive or the "recovery partion.". Booting into the recovery partition gives the ability to open disk utility from the menu bar.
Open disk utility to run disk first aid or "repair disk" to possibly bring the drive back online.

If that does not work,

Try booting into single user mode by holding down command+S. This brings you to a command line prompt. Now run fsck -fy or sbin/fsck -fy
This will either repair the drive or confirm that the drive has failed.
This image is a theme.plist hack