If a user has been able to open BC and suddenly they are not able to open BC. Check the following.
Search for Users.
Open the Users card with the issue.
Check the Permission Sets
if there is a Company Name in the list for all Permission Sets entries then the user only has permission to the Companies listed on the record. The example below the user only has permission to Cronus USA, Inc company.
Search for User Settings or User Personalization (BC version < 18.0)
Check the Company listed for the User if it is not a company that was listed in the permission sets then change to a valid company.