The router was still working. My desktop connects via cat-5 and that was still working normally, but the hardware had basically gone rogue. Rebooting it did not solve the problem. I reset it to the factory defaults and everything is back to normal now.
Notably, a wireless network that I could see broadcasting before I reset it now seems to be gone.
The likelihood that this was caused by some freak circumstance while I was gone seems too low to consider. However the likelihood that someone sat outside my house and spent the time brute-forcing their way through both the wireless network WEP and my administrator username and password--for no other purpose than to hijack my access point on a frolic--strains credulity only slightly less. Fing didn't show anything except my desktop connected to the network before I reset the router.
Should I be concerned?
Update (April 8, 2010): Turns out the landlord's family had reset the router while they were here over the Easter weekend to get the landlord on email.