Huge chunk of Hard Drive Free Space Missing

Hello all. I could really use some help. I have a TB hard drive with 917 GB of actual space. But right now, I only have 27 GB free. Why? Because SOMETHING happened and now 602 GB of space has gone UP in smoke. Cannot be found. I used WinDirStat and it just shows up a HUGE yellow box that simply says "C:" and leads to nowhere. I then used Treesize and it doesn't even pick up the...