Dell E6520 will not connect to wireless network

My E6520 has just stopped connecting to the wireless network it has been connecting to for two years. No hardware or software has changed, and other laptops and phones are still able to connect to the network. I can even connect to internet using my phone as a hotspot, just not through my regular wifi network anymore.

The message I get when I try to connect is: no network access. Or sometimes it just keeps trying to connect but never does, then says "connection was unsuccessful." Then it says I have "limited connectivity".

When I use the Network Troubleshooter, it says that it is resetting my wireless adapter. Once it has done so, it claims to verify that the problem is resolved. But it is not resolved. It sometimes says I have no preferred wireless networks, but I do. Or is says problem with wireless adapter or access point which it cannot fix. The answers change each time, but it never fixes the issue.

I have rebooted laptop, router and modem. I have verified that my wireless switch is turned on. Not sure what else to try. 

Thanks for any help.

Bert