If they were once able to see the tabs and information for your organization, they may have been removed or archived. Conduct a People >> Search and unarchive them. If they do not appear in your search, generate the signup link for your organization and send that to the volunteer, enabling them to re-join your organization (Logging in on the right, will let them re-use their existing username and password).
If you are able to see their account and it is not Archived, the volunteer may have more than one Better Impact account. Please verify the volunteer is logging in with the exact same username you can see on their profile.