Is ground beef that was in the refrigerator for two weeks and has turned brown still usable?
First, do not eat that. Regardless what color the beef is, two weeks is entirely too long to refrigerate ground beef. It is unsafe and should be thrown out. Raw ground beef only keeps in the refrigerator for 1-2 days.

Ignoring storage time, regarding color, brown meat is as safe to eat as red meat. As others have indicated it is simply oxidation occurring. Many grocery stores, at least in the USA, [used to?] blast and wrap their meats with carbon monoxide which imparts a bright pink color that lasts well beyond the shelf life of the meat. If you freeze hamburger or steak for a few months there's a good chance it will come out much browner than when you put it in. This is because some oxidation still occurs in the freezer, unless you hermetically sealed it somehow. Similarly, as long as you're within the 3-4 month freezer period for ground beef, you're OK.