Hi,
I have the same problem as _Pascal_ with the same symptoms.
Power BI Desktop
Version: 2.79.5768.721 64-bit (March 2020)
I found that this occurred after adding a new table and trying to create a relationship. The lines previously created disappeared although the relationships remained.
I then disabled the data load of the offending table, did a view refresh and the lines reappeared.
I then added the offending table back, made the required relationship but lines again dissappeared and after a view refresh the whole window greyed out (as per _Pascal_ screenshot) and froze.
I have not found a work around even though I rebuilt similar models in two different workbooks without reusing M code. There are no visuals and no DAX in the workbook yet.
Cheers Tim