Please can someone advise me on this?

I'm confused, all of a sudden I have this error message under the menu in admin

1034 Incorrect key file for table 'zen_customers'; try to repair it
in:
[select count(*) as count from zen_customers]
If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.

Nothing has been changed on the site. just one day logged in and this message was there....

???

Phil