If there have been no changes on the router or phone, then I would reset the router (power off,wait 3 minutes,power on).
Wait for router to fully come up,then reset the phone (power button and volume down at the same time).
Routers are notorious for losing their way at times.
DLink Routers have this common issue, Power off router for 30 seconds and power back on, Then scan with with your phone, Should find and connect.
I'm having the same issue, but with different wifi in different locations and I know it's not my home router because my other devices are not having issues