I can't tell you what happened but one suggestion that has helped me now and
then is to install wicd. Its in the repositories. You will need an internet
connection (obviously) so hopefully you have a wired connection available.

I have a HP Compaq that occaisonlly does the same thing. instead of
reboot, at top on the wireless icon, I right click and disable the
mobile broadband, then left click to start the wireless and it take a
couple of seconds and am back online.
cuzzin dick

I have a similar problem with my Acer notebook. I have found that logging into
the network with Windows restores the Ubuntu connection. I suspect that this
shows that the problem lies more in my home network than in my laptop, but I
don't know enough about networking to fix the problem.