fcpx suddenly closes

Just to make sure: did you get a dialog window and click on the Delete Preferences button?

If you did:

you may try downloading a fresh copy of the application

try running FCP X from a different user account. Same problem?

Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

Then you did not delete the preferences. Maybe your timing of pressing the keys was a bit off.

You should try that again.

With FCP X closed hold Option-Command as you launch FCP X.

This will allow you to reset the FCP X preferences and hopefully let it open.

BTW: what type of footage are you using?

Al