Hi Pete, and
I'm assuming you've tried the following:
1. Power off and disconnect everything;
2. Connect all devices together via Ethernet;
3. Power on the modem;
4. Wait 5 minutes;
5. Power on the router;
6. Wait 5 minutes;
7. Power on the computer.
When you're swapping from a direct connection to the modem to putting a router in between it and the PC you need to be sure to power cycle everything in the right order.
Can't be sure this is going to work, but it's worth a try.
If this doesn't work I'd suggest borrowing a router from a friend to see if you can get that to work, it could be that your router's just died.