Hi i have the same problem and i'm pretty sure its to do with DNS, for example if you type
www.google.co.uk into the address bar you will get a time out/website not found, if ur using firefox you will see in the bottom left of the window "looking up www.google.co.uk" basically its trying to do a DNS lookup for the ip address for the site, however if you type if the ip address of the website [if you know] you might well find that you get access to it..why? 'cos the IP address is more or less absolute it doesn't rely on DNS.
If you go into a DOS cmd window and type ipconfig /all that will display all of the ip info for the local pc/laptop if ur using a router the router ip will show as the gateway,the dhcp server and the DNS server, the router will/should be configed to get DNS setting automatically from the ISP so far i've yet to get the the reason for the loss of DNS, does anyone know whats virginmedias DNS server IP address are? if you do the you can force these onto the router.