When your mac starts up to a folder with a question mark in it this means that either the startup disk has damaged or the macOS operating system on your startup disk is damaged.
Follow these steps in order to fix this issue:
- Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
- Every Mac has a power button. On notebook computers that have Touch ID, press and hold Touch ID.
- Use Disk Utility to repair your startup disk. Follow the steps for a Mac with an Intel processor.
- If Disk Utility found no errors or repaired all errors, reinstall macOS.
- If you still need help, please contact Apple Support.
Also, see this link for further reference about this issue
If your Mac starts up to a question mark - Apple Support
Hope this helps!