Yellow Other Bar!! Why?!

So something is wrong with my computer and I need your help! just a couple of hours ago when I checked my storage it was pretty full but not completely - it showed what was allocated to audio movies photos etc. Now that I restarted it that storage is entirely yellow with the "other" category being the only thing reflected. I have emptied trash and downloads to create space but it's still all yellow. What happened to the individual categories? Both before and now I was getting the message of "start up disk almost full". I read some other posts and downloaded that omnidisk thing. But I don't know how to find those files ... I am not very computer literate for things like this. I also don't know how the change went from compartmentalizEd to just other

Help!!!

THank you!!

oh and I have os X 10.8