Is everything you try to do taking a long time (ie open programmes etc) or just internet access?
If it is the latter it could be your isp. I have found that when these things happen 'out of the blue' they will rectify themselves in the same manner.
If you had not made any changes immediately prior to this I would suggest you wait a while to see what happens. You could call your isp but probably won't get an honest answer. Having said that it would at least alert them to the problem, if they were not already aware of it.