I tried to plug an ethernet cord into another device, but it too went unrecognized, leading me to believe it the the modem. I factory reset the modem and the problem persists. I really don't know where else to take this.
If this router/modem is leased, you can get a replacement through your ISP. If you own it, you will have to buy a new one.
I lost a router in a similar way, but it wasnt a router/modem combo. I had a Netgear that died in which none of the devices connected to it would uplink even after reset of power as well as reset of the router back to factory. The one thing I did notice though is that the LEDs were not as bright as they use to be, and the router was very warm to the touch. In addition to this, the power adapter for the router was also warmer than it should have been. I replaced it and problem solved!
The good thing though is that I had a spare router on hand to switch back to the 10 year old Linksys which didnt have wireless and still had settings for Vonage and port forwarding etc, so I was able to just set this up for a week until I could get my replacement for $20, a D-Link wireless router from newegg. I placed my 10 year old Linksys back in the box in the closet in case I need to use it as an emergency router ever again.
We had a bad power outage prior to mine dying. I had it accidentally plugged into unprotected power. We had the lights go dim, then bright and then out and then back on for a second and out and it did that about 2 times and then finally out for 30 minutes of no power. When power came back on, the router didnt make it through the abuse without power protection. Everything else powered through power strips with surge protection and battery backups were fine. This was plugged into an older power strip that didnt have surge protection that my lamp was also plugged into and of which the florescent bulbs ballast was blown. Threw away that power strip and paid $7 for one with surge protection!
Have you had bad weather lightning etc or power outage etc that might be linked to this problem?