Hi ​@Barbara, welcome to the Box Community!

It’s possible that your collaboration invite has expired. To regain access to the files, please contact the file owner and ask them to re-send the invitation.
 

If you've regained access but are still experiencing issues uploading files or folders, the following troubleshooting steps often help resolve problems with the web app:

• Make sure that your web browser is the latest version of Chrome, Firefox, Safari, or Edge. An out-of-date browser can cause file upload problems.
• Clear your browser's cache.
• Confirm you're using one of our supported operating systems.
• Check your network connection to Box.
• Temporarily disable or configure your firewall to allow Box as a trusted site.

For more detailed guidance, visit our Troubleshooting Uploads to Box article.

If you have any other questions or need further assistance, feel free to reach out.