Too often have I seen cashed-DNS problems, especially after a network maintenance period, IP update, or even domain transfer - where the client's think their websites are down.

This website will tell you if a website is down, or not: http://www.downforeveryoneorjustme.com

Another useful website to use, to check if a website is working like it should, or if the updates you have applied is there (but your PC or ISP's cache is not showing the changes) is http://www.proxify.com