There are a huge number of things that can cause sites not to load. Could be a problem with the site, with your computer, or anything in between your computer and the site. See what this site says: http://www.downforeveryoneorjustme.com/ That'll possibly rule out the site as being the problem.

If it's a problem with your ISP, there's nothing you can do but wait. If it's your computer, maybe you can figure something out. First thing to try is reboot your computer. Then maybe try a different web browser temporarily, maybe yours has a misbehaving plugin or something. Then there are a few things you can try to gather information from there.

Given it's working again, probably pray to the internet gods that it keeps working.