Sounds like you continued to model from the second table in your join after you added the join to your data model. That would mean that the second table is a temporary table for another active table. Find the data flow for that "other" active table, and select Add table to worksheet.
Then you can easily join the two tables again.
Let me know if you have trouble finding it.
P.S. Thankfully, you should rarely lose work using Xcalar Design. The main risk would be you specifically drop the exact table you need. In that scenario, you should be able to model any dropped steps again. Take care, @vhall.