A tricky one, this is a phrase with lots of meanings.
I guess in your context it means you fail to complete a task on time, or make no preparation for an important event.
表演要开天窗了 : The performance's screwed up (because the actors make no preparation or someone forget to bring something important for the performance.)
and you can also use this phrase when you are not well prepared for your presentation.
You can also see the phrase in the idiom:打开天窗说亮话
it's like "let's get it straight", but the subject is usually unpleasant.