YouTube and Google won't load!?

YouTube and Google won't load and it's getting really annoying! It says "this webpage is not available..because the DNS lookup failed?" what does that mean, I have no idea was a DNS lookup is. I tried restarting my computer and clearing all my history everything, but it doesn't work. If anyone knows what to do, please help me! I really want to be able to go onto Google and YouTube again :(