The prices do not double overnight for all the images. There are usually two explanations for price modifications:
1. The images may have moved to a different level during those hours. It is however a bit unlikely for all of them to have changed level. Our image prices depend on the number of the download images get. The more downloads, the higher the prices. See the table on the page below for a complete list of prices:link
2. The image prices may have been discounted for 24 hours. This is a promotional offer we give to new users but the downloads need to be made within 24 hours after registration. You can read more about this here:link here
Most likely this is what happened in your case.
My strong recommendation is to contact support link here
. They will give you a solution for the problem.