Unable to access to shared files

Hi Fran, 

Welcome to Box Community and glad to assist!

Did you check if the email address that was used to invite you to their file/folder is the same with your email on Box? 

If yes, look for your pending invitation and accept: 

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

Otherwise, please contact the file/folder owner to send a new invitation. 

Thanks for posting!