Can't log on to PS network? PS4

I've been unable to acsess PSN for about two weeks now. It always used to work before.
When I go to "Test internet connection" it says I'm connected to the wifi (72% strength) but PSN sign-in failed.
I've tried restarting the PS4, and the modem. I honestly don't know how much else I can do. 
It would be great if someone could give me a few tips as to what I could try. 

Thanks in advance.