It could be a problem of the driver or of network manager. In the latter case, try to see if
% sudo restart network-manager
works. If it works, you can add the network-manager restart at resume: there are a lot of hints and suggestions here: how to execute a command after resume from suspend?
Otherwise, you should try to load/unload the wireless driver --- you can find it with
% nm-tool | grep Driver
(you will have the list of drivers, one of them is you wireless one), and try to unload and reload it:
sudo rmmod ath9k
sudo modprobe ath9k
If this second option does work, you can add the modules to the list of modules to be loaded/unloaded at suspend/resume,
% sudo sudo gedit /etc/pm/config.d/config
there. (Disclaimer: it worked for me in 12.04. Never tried with newer Ubuntu).