Next time you have this problem, try
sudo 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 16.04.
If you are having more severe Wifi dropping issues on Ubuntu 16.04, this page has discussion of deeper fixes to Ubuntu 16.04 Wifi dropping.
Canonical bug report for NetworkManager Wifi not working upon resume from suspend or hibernate in Ubuntu 16.04.