Hi Megan, 

Welcome to Box Community and glad to assist!

Kindly reach out to the folder owner and request for a new invitation for the affected folder. 

This is the alternate way to accept invitation:

1. In the left menu of your Box account window, click All Files.


2. In the pending invitations panel under ACTION REQUIRED, hover on the name of a file or folder.


3. To see details about the invitation, click the ellipses, "...", and select View Details.


4. To decline the invitation, click the ellipses, "...", and select Decline All.


5. To accept the invitation and gain access to the project's content, click Accept.

Thanks for posting!