Magnetic Storm
As a Radio Ham (no doubt Hancock jokes will follow), the last time we had a big magnetic storm in 2011. LF to HF radio went down for about half an hour.
On VHF-UHF the opposite happened with better propagation between stations in the higher latitudes, just after the storm.
As for sat nav, we will have to see what happens. When I am sailing, I do not use sat nav. Give me a chart, compass and a stop watch any time.