Error 404

Listen up, buttercup. This joke is all about Stephen Hawking and his wheelchair. The joke plays on the stereotype that Hawking is a machine and that no one is home. Like you, the author probably thinks error 404 is something bad, which is a tech term when a webpage can't be found. It's funny because it suggests Stephen Hawking is missing, or his brain is malfunctioning. It's basically saying his "page" or mind, is unavailable, get it? Also, it's Hawkins, not Hawkins'.