sudo ifconfig

will tell you whether the adapter has an ip address allocated

dmesg | tail

might shed some light on what caused the connection to drop

sudo dhclient -v wlan0

dhclient will request a new dhcp lease, if your network adapter is configured to use dhcp. This worked for me, although it took some time. AFAIK, dhclient will keep on trying to acquire an ip address until it gets one or Ctrl + C is pressed. It might take a few minutes to succeed!

Other things to try

If the above doesn't work, you could try the following:

nmcli networking off 
nmcli networking on

restart network-manager:

sudo service network-manager restart

reload the kernel wifi module:

sudo rmmod iwlwifi
sudo modprobe iwlwifi

The module name will depend on the wifi hardware installed. To find out the module name try either of these commands:

lspci | grep -i network

or

sudo lshw -C network

and look for a line containing driver=something

*-network
   description: Wireless interface
   product: Wireless 7260
   configuration: driver=iwlwifi latency=0

Hopefully one of these methods will work. Otherwise check out these resources:

AskUbuntu

• http://askubuntu.com/questions/271387/how-to-restart-wifi-connection
• http://askubuntu.com/questions/54710/how-do-i-reset-the-network-adapter-using-a-terminal-command

Stephen Ostermiller's Blog

• http://blog.ostermiller.org/resetting-wireless-networking-on-ubuntu-without-rebooting/