Why is the speed of my internet connection suddenly so slow?

Our family pays a lot each month to have a decent internet connection speed, which I believe is 15 Mbps download and 2 Mbps upload. However, the download and upload speeds have abruptly lowered to beyond acceptable levels without any apparent reason. We do use a wireless router and have had no issues up until this morning. It is now taking more than a minute for a 6 MB file to download. What is the cause of this issue and how can it be fixed?