have you done a complete system update making sure that your software sources download location is set to "Main server"? (Or the next major server in your region!):
Which version of Ubuntu MATE? I'm having issues with the Wi-Fi causing errors and "cutting off" an awful lot after upgrading to 15.10. Mine is a Netgear WN111v2 USB Adapter.
As a temporary workaround, try restarting the network manager, that seems to do the trick for me:
sudo service network-manager restart -- or -- sudo service NetworkManager restart
How To Fix No Wireless Network In Ubuntu 14.04 ,14.10 and 16.04. Here i am sharing my experience .I always got error when i reinstall ubuntu or install fresh ubuntu installed. When you install ubuntu , every one face two common issue touchpad or wifi related isuue.
it Working For me.