I had Ubuntu first, installed Windows later, and for a short period of time I could choose the OS to boot into from the Windows boot, and now it doesn't even give me a choice, just boots right into windows. I have about 200gb of data on the Ubuntu partition, and I need a way to boot back in. How can I resolve this issue?
You will need to re-install Grub. Every time I installed Windows AFTER Linux, Windows would write its Master Boot Record (MBR) over Grub, therefore you will need to re-install grub on your drive.
I don't remember the command from Grub, been too long, hopefully someone can help you out with a link or a step-by-step guide.
Every time I had this issue, all of my data was safe. Unless you did something wrong or something went wrong, all your files should be fine.
Sounds to me like you installed Windows over your Ubuntu install or killed grub.
Windows doesn't play nice with Ubuntu/Linux, but Ubuntu Linux does. So most the time the best way to do this is to install Windows then Ubuntu, but no worries you can easily repair it.
You can watch this video to get an idea http://www.youtube.com/watch?v=ZcbTgMKpVHQ
To simplify you can easily just create a live distro and do a grub repair.
Hope this helps.