Since you now have an interface listed, can you ping anything? Can you ping
itself?

If the IP is statically set in /etc/network/interfaces, rebooting certainly
wouldn't hurt, but it shouldn't be any different than stopping and starting
the interface. If you want to go a step further, do "/etc/init.d/networking
restart" instead.

Afterwards, check the results of ifconfig eth0. If the listing for the
interface is ok, then try and ping the gateway IP. If that is successful,
well you're in business and I would look to your DHCP server. The DHCP on my
linksys has stopped working a couple of times over the past few
years...power cycling usually fixes this. Or it could be as Jeremy
suggested.

If you cannot ping anything on your network, then try and ping the IP
assigned to the interface. If you cannot ping that (eth0 not lo), then there
really is a deeper issue.