Check out this thread: https://developer.salesforce.com/forums/ForumsMain?id=906F000000090dFIAQ. It seems like you have a model where you’re query is not selective enough. You should use conditions to make the query as efficient and targeted as possible. Do you have a lot of models on the page? What object s are they on? What conditions do you have on the models? If you give some more information I can try to help.

Sounds like one of your Models has a lot of Child Relationships in it — do you know if is this true? Did you add additional Child Relationships since last week, or was the page unchanged since last week?