First things first: Don't panic!
You can always download a live image in order to recover all of your files. Various Linux distributions are available as a live image, including Ubuntu. These live images allow you to try out a Linux distribution without installing anything on your hard drive, which can be a great tool for data recovery.
There's a bunch of ways to get a live image bootable on your USB flash drive.
Depending on the operating system you have available, these ways vary. Fortunately, Ubuntu's official website provides well-documented tutorials for
Mac OS X and (somewhat ironically)