After fighting to get back online after persistent connectivity problems, I've stumbled across the dummy solution: Network Diagnostics. Apparently that resets the connection more completely than simply trying to renew the lease within the Network settings. (One easy way to get it is to use Safari to call up a page. I know I know....)

Of course, if you're reading this, then you're online and not having any problems. Maybe you can pass this along to anybody finding themselves suddenly locked out.

(I'm sure there are more geeky ways to reset connections using *nix. Alas that's beyond me.)