As a techie ignoramus, the last time that one of our family lappies refused to recognise the modem/router, I took the lappie to the modem router, attached required cable, did not a lot and it worked. Then, having looked at the domestic network through that link, after a while - patience required - and patience is not my strong point -, said router acknowledged the lappie as both a hard wired and a wifi goodie. Lots of turning things on and off required. In fact, the router has a memory much better than mine about what has/has tried to connect........
In the end, lappie did respond. (All were configured with the same security), but lappie had forgotten security......
Which is bloody wierd as junior daughter's lappie is seldom here, but has a great memory for this stuff.