Problem with View Field

I have a view field in one table that shows the records from a second table. When I click on one of items in the view the form for the other table comes up but there is no data in any of the fields. 

This has been working for several months but yesterday it failed although nothing had been changed. I'm wondering if anyone has had a similar problem. 

The only clue I have is that the select statement used in the View field is a bit convoluted. However that seems to work as it displays the records from the second table.  It's only when I select one of those records that the form for the second table has no data. Maybe it's some corruption of the data or I have somehow lost the link into the other table.

Grateful for any help.