Disappearing Linked Table

Question

I have a problem that arose last week and seems to be spreading. I installed a MS Access database front end on several pcs at a company, that link by ip address to SQL Server backend. This was done over a year ago and has been working perfectly until last week. The frontend accesses the user table in the back end to enable the user to login. It came up on one pc that the user table was not a valid name. When I looked at the database, the linked user table was missing from the linked tables in the front end. I put a fresh copy of the front end on the pc and when you try to login you get the same error and you can actually see the user table disappear from the frontend. If I go to the LTM, to add the table back again, it is not showing in the available tables from the backend. I have tried several times to put a fresh copy on the pc but the same thing happens every time. Today, 5 computers in the same office have the same problem. If anyone has any ideas, I would be most grateful.