Interesting one that. I had exactly the same yesterday, scratched my head and it turned out that the ethernet lead in the router was a bit loose, It may be worth checking the connections.
I’ve checked the Ethernet connections they seem ok. Nothing had been disturbed before it went off or touched before it came on again and then after about 2 hours it went off again with the loss of the tv light on the router
I think I’ve solved the problem now. I use a powered Tp-Link Ethernet switch to connect to the talktalk box ,the tv & an android box . It seems to have gone faulty, I unplugged it’s power chord and reconnected and the router tv light came back on , it’s still working now so all seems ok. Thanks for suggestion of checking connections etc.