Ubuntu since version 13.10 has a nasty bug: After waking up from sleep, the network is disabled and does not automatically restore.
The corresponding Ubuntu bug #1252121 has 131 comments but no fix.
A workaround is to create a script that gets run after the computer wakes up. Put it into /etc/pm/sleep.d/01_restart-networkmanager:
#!/bin/sh /usr/bin/nmcli nm sleep false
Then make it executable with
chmod +x /etc/pm/sleep.d/01_restart-networkmanager