Next time you experience something like that, simply type 'is site down' in your search engine, which should bring up a few results and on one of these you can paste in the url of the site that won't open.
You will then be told something like 'yes looks like it's down for everyone' or alternatively 'looks like it's just you'.
If it is the latter answer, have a clearup with Ccleaner, after which the site will probably open.