I have a similar Linksys system, but I'm abysmally ignorant when it comes to even vaguely understanding how remote wireless connections work.
Except it does seem whenever I've had a problem it usually was solved mostly by various combinations of plugging and unplugging and rebooting and letting the automatic parts of the system find their connections.
First try unplugging the router and the slave for a minute or two and then reconnect them to see if they can work things out. If not, maybe unplug them both and reboot first the upstairs computer, then the downstairs computer, then reconnect the wireless links.
Sorry I couldn't be more helpful, but plugging and unplugging is worth a try. You've probably already done this, but rereading your equipment setup instructions might help too.