Hi there.
This has happened to me also. I'm developing a woocommerce based website and I have many products with variations set and working. Today I updated all my plugins and all the variations are missing like described by Joelseq. The only way I have found to make the variations appear in the front page in the dropdown menu is to resave the product. One by one.
In my stagging server I've Woocommerce 2.6 installed, and I've noticed that this problem also exists.
Something happened in the database upgrade from 2.5 to 2.6.x that affects variations.
Woocommerce should have a bulk edit for variations, so one could edit a set of equal products in one go. This would avoid too much manual work, editing products one by one!