After a tough week of battling a seriously impaired site, I wanted to document my error and solution since I didn't find any specific tips in the forums about this.
After installing several unused modules, I started getting this error message
Table 'database_name.node_field_instance' doesn't exist query: content_schema /* Anonymous : content_schema */ SELECT * FROM node_field_instance nfi LEFT JOIN node_field nf ON nf.field_name =nfi.field_name WHERE nf.active = 1 AND nfi.widget_active = 1 in /home/kbnet/public_html/sites/all/modules/cck/content.install on line 187
This resulted in content fields for several content types no longer being accessible (including embedded video, ckk email contact, and calendar dates).
I compared the variable table of the drupal database with a backup and found that this row had been removed
I inserted it using phpmyadmin (old value was i:6009; in my case) and that solved the problem.