I would look at it not as, "Why has my normally working router suddenly broken?" but rather, "If I had a different brand of router I may not be able to recover it at all, phew".

I agree with you absolutely.
I have used nothing but Draytek for many years, starting with 2600. I now have a Vigor is 2800n, which links to fibre broadband.
The one that went faulty was a client one, 2800G, which he has had for many years. It has served him well.
I have replaced it with a 2600G (temporarily). I will now return his 2800G. He is in a very bad area, so an upgrade wouldn't do him any good.

He is moving to new premises where I am installing a 2800n. This will be serving 5 computers (networked) and access to the Internet.
Doing all this next Thursday.

He is based 15 miles away, so I use VNC to access his computers, except when the router dies...

Thanks again for all your help.

John C