CryptoLocker will silently encrypt ALL of your data (including photos and videos!) not just on your main disk drive, but also on any additional system/network drive letters, and possibly on certain cloud drives you may have. It will then present a ransom demand for $100-$700 which must be paid within 4 days, or the decryption key will be erased.
As you'd be dealing with crooks, it's NOT recommended to pay the ransom as there's insufficient evidence that paying ransom actually works (is there honor among thieves?). The alternative is after removing the virus, you have to restore EVERYTHING from backups, making CryptoLocker MUCH more destructive than previous "fake FBI" viruses.
If you've already been 'encrypted', disconnect the computer from the internet, unplug any attached backup disk drives, and contact Professional Nerds right away.