In the last few days some of the high speed internet connections (from different ISPs) which I use have become very slow and the reason is a cut in the international submarine cable SEA-ME-WE 4 in the Mediterranean Sea last week.
Based on this report at http://www.itp.net/580051-repair-work-on-damaged-cable-delayed-by-a-week, it appears it will take at least 2 more days for the cable to be repaired.
Usually cable cuts are the main reason why usually high speed internet connections become slow. If just a single broadband DSL / cable connection is slow or non functional, it is a problem with the local cable network as the cable is cut during digging by other utilities. If both the wireless and DSL/cable broadband connections are slow, it is usually a problem with some undersea cable far away.