I have a couple of laptops that do this from time to time, and so does my phone. Not sure if it's to do with the order they scan WiFi channels in or/and which band (2.4 or 5 Ghz) it scans for first, but every so often they lose one and it takes a while to get it back (or a restart). My guess is they get confused which of 2.4 or 5 Ghz to use and pick the wrong one.