The DNS server is a computer (usually at your ISP) and when you first set up your internet connection they either gave you an IP to type in or maybe your router gets one from your ISP automatically.
It acts like a phone book so when you type in www.mumsnet.com or bbc.co.uk your computer calls this DNS server and asks it what the IP number for that is so it can connect.
They go wrong sometimes or too many customers are using it so it gets overloaded. It's nothing you did and mostly if you wait your ISP will sort it out.
Some ISPs are slow to correct it so people tell their computer to use another one for the time being. Lots of people use these OpenDNS ones for example.
208.67.222.222
208.67.220.220
They come in pairs like that. A main one and a backup one and you put them both in.
Below is how to tell your PC to use these if you continue to have problems and want to give them a try.
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In XP (other systems may vary a bit)
In Control Panel open Network Connections. You should see an icon in there for your internet connection. Right click that and select select Properties.
Now you should see a box with some lines of text and the bottom one says Internet Protocal (TCP/IP)
Double click that to open up another box which near the bottom says 'obtain DNS server automatically'
Tick 'use the following DNS server addresses' and it will let you type these two iP numbers into the boxes below.
If there are already numbers in there those will be the ones the ISP gave you. Save those in case you want to change back.
If it's empty then changing back means simply ticking the 'obtain DNS server automatically' box.