As a rule of thumb it's safe to say that if you did not make any changes on your website, your webmaster hasn't and your hosting company hasn't then someone has hacked your site. Websites don't usually just stop working for no reason.
Some of the most common flags to the issue are:
- Nothing showing up
- There are ads for sexual performance drugs on your website, or
- You've gotten an email from their hosting provider saying you are sending out spam.