If Ubuntu won’t allow WiFi connections after waking from suspend, try
systemctl restart network-manager.service
If that works, see the discussion of minor script modification to auto restart NetworkManager on resume. If it’s a chronic problem, those scripts will run this command for you at each resume to make your Wifi work upon resume from suspend on Ubuntu.
If you are having more severe Wifi dropping issues on Ubuntu, there is discussion of deeper fixes to Ubuntu Wifi dropping.
Canonical bug report for NetworkManager Wifi not working upon resume from suspend in Ubuntu.