The most convenient way that you can follow to install network-manager back is to install it in the livecd, if you have one.
Insert livecd and boot it, and connect to internet. (If you can not, there is a problem with your network)
open gnome-terminal, type in following commands
This one lists your partitions, find out the partition number of your root partition, which should be flagged with 'linux 83 '. In the following part I suppose it's 'sda5'
This one mounts your root partition to /mnt
sudo mount /dev/sda5 /mnt
This one chroot current / from the livecd root to your physical system
You may also need to install several other relative packages. Then exit gnome-terminal and reboot. Your network-manager is back.
sudo apt-get install network-manager
Or else, you have to use command line to establish a wireless connection. I never type that myself, you need more googling.
Originally Posted by patilsb