how can i open my mac? question mark is flashing

Your startup disc can't be found. See Apple's support document If your Mac starts up to a question mark - Apple Support:

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system

If the question mark is persistent, and your Mac does not start up normally:

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 found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support.

When restarting your Mac make sure you have followed the correct restart procedure for your type of Mac, either Apple Silicon or Intel. They are slightly different. After restart your Mac will open to Disk Utility - see below.

Determine whether you're using a Mac with Apple Silicon, then follow the appropriate steps:

• Apple Silicon: Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labelled Options, then click Continue.
• Intel processor: Turn on your Mac, then immediately press and hold Command (⌘) and R until you see an Apple logo or a different image