Apparently someone at the Russian space agency goofed and uploaded erroneous ephemeris (orbital position) data to the 24 GLONASS satellites on April 1. Without good ephemeris data the satellites don't know where they are and you can't use them to find out where you are. The General Lighthouse Authorities of the UK and Ireland (GLAs) released an annotated chart showing the extent of effects caused by disruption to GLONASS last week.
The outage lasted for more than 10 hours on April 1/2. Russian GLONASS operators had to wait until each satellite was over ground stations in Russian territory before the operators could upload corrected ephemeris data.
Nick Ward, Research Director at the General Lighthouse Authorities, said “We have known for a long time that satellite navigation systems are vulnerable to interference and this event demonstrates just how dangerous a failure can be. Our data shows that, during the prolonged disruption, GLONASS was providing location data up to 55 kilometres [34 miles] out off the UK coast. The potential implications of such failures are severe. A vessel traveling in coastal waters, for example, could face vastly increased chances of grounding or collision, and ultimately loss of lives and cargo, as well as pollution. Such a problem could occur with GPS, Galileo, or any other GNSS without any warning."