can`t open contacts. iMac, high sierra.

Greetings, 22379000025298.

I see that the Contacts application isn't opening using your iMac on macOS High Sierra, and I'm happy to help.

How to turn on or turn off your Mac -- Restart your iMac, then test the issue.

Use safe mode to isolate issues with your Mac -- Restart your Mac in safe mode and test how it responds. If there are no issues, reboot normally and test again in your own user account. Remember this when booting in safe mode: Limited graphics performance in OS X recovery or safe mode

How to test an issue in another user account on your Mac -- If the issue is also happening in safe mode, use this article to create and test a new user account. This will demonstrate if the issue is isolated to your account, or if its system wide.

Let me know how that goes, and take care!

Try restarting the computer. If that does not work, please read Writing an effective Apple Support Communities question