Mark writes in wondering why his 2013 MacBook Air's function keys won't perform the functions on the labels--they only work as, er, function keys! Some software either assigns commands to function keys or allows you to set them.
But Apple changed the behavior a few years ago so that instead of the default being F1, F2, and so forth being interpreted as those keys to the system and software, OS X instead uses what were alternative functions, like increasing or reducing brightness and volume.
Fortunately, there's a simple solution, if this is the problem:
Open System Preferences > Keyboard.
Check the Use All F1, F2, Etc. Keys as Standard Function Keys.
If that doesn't solve it, it's possible you'll need to reinstall OS X, which you can do without overwriting your current system. Follow Chris Breen's Yosemite installation instructions, which include working with an existing OS X system.
Not without my data
After recent exploit discoveries, government database hacking (this time, the government's, not by the government), and the LastPass data theft, it's germane to answer George Barnette's concern about data ransoming:
I read about people who suddenly find their drive's contents held for ransom, and assume they downloaded or opened something they shouldn't, and further, that they're PC users anyway. If one diligently uses a backup, such as Time Machine, can one ignore the extortionist, or has one simply backed up the cyber-bomb as well as one's data, so that reconstructing the drive from the backup reloads the bomb, as well?
I haven't heard of this happening to a Mac user, but it's not outside the realm of possibility that someone would download and install malware using administrative privileges that did such a thing. Mac users might be generally immune so far from serious attacks, but downloading and installing unknown software is an easy route, should a malware designer believe they can convince enough people to do so.
A Time Machine backup drive, unless it was a network drive, could also be encrypted by ransomware, and thus be at risk. A networked Time Machine drive would let you restore your computer or the data to another machine, but unless you knew exactly when the malware was installed, it could still be there, ready to be triggered again.
In such a hypothetical, I'd retrieve my known applications and documents, but not perform a full restore. This is also a good argument--even as an extreme case--for having offsite or cloud-hosted backups in additional to local ones.
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