Click in the Macintosh HD partition in the partition map. You should see a triangular gadget in the lower right corner of the active partition as shown below:
Select the gadget with your cursor and drag it to the bottom of the space, thus, extending the size of the Macintosh HD partition back to its full size. Click on the Apply button and wait for the process to finish.
There is no Recovery Mode. There is a Recovery HD if you run Lion or later, and need to boot from it in order to make changes to the startup drive.
Boot Using Command+R keys:
1. Restart the computer.
2. Immediately after the chime press and hold down the
"COMMAND" and the "R" keys together.
3. Release the keys when the Utilities Menu appears.
To open Disk Utility simply select it from the menu of options then click on the Continue button. I think you may need to try doing this:
Click on the Add [+] button. This should create a new partition in the Empty Space. It should be possible, now, to change the sizes of each partition. If so, then select the bottom partition and click on the Delete [-] button. Now see if you cannot expand the upper partition to its full size. If you still cannot do this then the drive is too corrupted to do anything with but re-partition it from scratch.
Does your computer happen to have a Fusion Drive installed?
That didn't seem to work. I was able to add a new petition but not actually make a new petition (when I would click create, Disk Utilities would not really do anything) and I couldn't remove one either.
No fusion drive as far as I know.
So basically my hard drive is corrupted? I'm assuming this also means I'll lose all of my data?
You don't have to lose your files. You can first make a backup before repartitioning and reformatting the drive (if it's salvageable) or replacing it (if it's failed.) I'm assuming you can still boot into OS X, so it won't be difficult making a backup. Besides, if you don't have a backup, then you need to have one.