In order to find out a drop in impressions from the Summary trend, you need to first choose a date range where you want to investigate for a drop. Eg: choose the last 7-days from the date range filter and then plot impressions from the summary graph dropdown. In case the trend for the impressions line is continuously decreasing, there is a drop.
What should you do next?
Plot spends on the summary graph. If the trend for spends is decreasing in proportion to your impressions then check the FAQ – How can one investigate a drop in spends?