Why my macbook is showing questions mark

Follow all four of the steps in If your Mac starts up to a question mark - Apple Support.

If your Mac starts up to a question mark this means that your macOS startup disk is not avaliable or that it does not contain a valid macOS version for your device.

To fix this, follow these steps

1. Press and hold the power button on your Mac for up to 10 seconds until your Mac turns off.
2. Every Mac has a power button. On notebook computers that have Touch ID, press and hold Touch ID.
3. Use Disk Utility to repair your startup disk. Follow the steps for a Mac with an Intel processor.
4. If Disk Utility hasn't found any errors or has repaired all of the errors, reinstall macOS.
5. If you still need help, please contact Apple Support.