JoleneP I hope we can help you resolve this issue as soon as possible. You might have already tried some of these, but here are a few things to check:
Try enabling debug mode and reproducing the same error. A debug button should appear alongside the error, giving you more detailed information.
Check your file permissions.
If you have access, try clearing the cache and running asset publish commands via SSH to see if any errors occur.
Inspect the database to make sure no core columns or tables are missing.
Check server resources such as disk space and other statistics.
Look through Nginx/Apache logs to see if anything relevant appears.
Check the network tab in the developer console to see if there are any API errors.