As always, it depends. The right mitigation depends on the situation. In your situation (home user), I'd say just overwrite the firmware with the latest original version and reconfigured everything with a administrative password in place.
During configuration consider things like "Strength of Wifi and administrative password? Is uPnP necessary? Or WPS? DMZ? Do I use WEP/WPA instead of WPA2? Setup a scoped Guest network? What Firewall options do I have?"
Back to basics
Make sure it's the original firmware you've downloaded (using a checksum check). That basically ensures that you have the original firmware running and not another version with a backdoor.
Risk (likelihood / impact)
Was it a router in a highly confidential area? Then, I'd calculate the risk based one the likelihood and impact (possible damage) and possibly consider to throw the router away and setup a new one.
Also keep in mind that not only the router here is the potential risk. Make sure you audit the configurations and scan all your devices in the network accordingly.