The most probable cause for this is that one or more of your campaigns have registered the first sale. When you have a campaign without sale than your ACoS is zero and does not count into the overall ACoS calculation.
The moment this campaign registers a sale the ACoS show the percentage. This could be a high value depending on the amount you spend on this ad campaign already. Let’s say you have spent already $20 on a campaign and get your first sale of $4.95. The ACoS would show almost 400%!
Keep in mind that sales are delayed by to 7 days. It’s normal that you see a higher ACoS, which will decrease over time.