Hi
Have you moved any of your module folders?
This seems like the uc_cart module is not enabled.
From your comments, you say it is so perhaps Drupal can't read or find the file uc_cart.module?
Have a look in your system table in the database to see the path Drupal has recorded for the uc_cart module, make sure the module is actually there and can be read by the web-server.
If the folder/module was/has been moved - you can force Drupal to re-discover the module by going to the modules page as an admin user.
Lee