Lumpy white chocolate mousse.

Does your recipe by chance call for gelatin?

As Nancy said- seeing the recipe will allow everyone to give you more useful help in deciding what might have gone wrong. Aside from that, I wonder if the problem may lie in whether you used actual white chocolate or white chips/candy wafer? Real white chocolate contains cocoa butter, where the chips are made using vegetable oil. That can make a big difference in how they work in recipes.

Please send a link to the recipe. And/or tell us if you've made any changes when you cooked it this time.
Then we can know more what's going on and what perhaps to suggest.