Keyboard mapping corruption issue perhaps? Check Tools / Options, Environment / Keyboard. Should see a drop-down for your Keyboard mapping scheme and next to it a Reset button. Hit the reset button.
I'm not responsible for this screwing with your settings - save them prior to doing this just in case you screw the pooch.
Oh, in case the narcs out there care, I think this is a valid question and would be put out if someone voted to close it. Of course now that I've said that, its a certaintity, isn't it?
If you have other key bindings and dont want to reset the lot use: Tools -> Options -> Environment -> Keyboard
Then find Build.BuildSolution or Build.RebuildSolution (on your preference) select Global in the Use shortcut in dropdown, place the cursor in Press Shortcut Key and slap F6. It'll show you what it is currently assigned to, for me is was to traverse split panes (maybe it overrides F6 when you use split panes for the first time? I've always wondered why my home development machine built with F6 and my work one doesn't and come to think of it I've never used split panes at home) then hit Assign.
I had a stale build in my solution (included libraries were apparently using old DLL versions and I was getting MissingMethodException's at runtime) so I deleted all the \bin directories in my C# projects, restarted Visual Studio, and suddenly this issue is now affecting me for the first time in the 3+ years since I started using Visual Studio.
No clue how this could have affected my user settings, but it apparently did. They went from using the "Visual C# 2005" keyboard profile to "default"