My MacBook can't connect to any wifi network

My MacBook has never had any problem before but suddenly it stopped connecting to the internet (on any network -- and my other devices are fine with these networks). It is also not discoverable by airdrop though it seems to be able to send. The wifi appears to be connected, but Safari, Firefox, App Store etc. can't connect at all. I tried shutting it down but it made no difference. The MacBook is 10.13.3, from about 2015.