Everytime I sudo update in the terminal, it gives me this message W: Failed to fetch http://ppa.launchpad.net/pmjdebruijn/darktable-release/ubuntu/dists/utopic/main/binary-amd64/Packages 404 Not Found. And whenever I use software updater it says Internet connection problem, when actually my Internet is perfectly fine. How do I fix this?
That is because the person who maintains the PPA has not yet made a package for Ubuntu 14.10 Utopic Unicorn, which is what you have. All you can do is remove the PPA.