Hard drive problem

Hello,
I have a 250gb hard drive, not my computer runs great and start fine, but one of this day I was trying to load data from my external drive to my computer and it said not enough memory, so i want into my computer and check the c: drive and now computer have a d: drive for back up. But that not the problem, the problem was when I look at the memory storage it only had 50gb or room, now i check the computer and it label 250gb, i open the computer and it said on the hard drive 250gb but why is my computer only showing 50gb and not use it full use, and no i dont have 200gb of ram. I did restart the computer once, before that when looking at the memory space it did have 250gb...what could be the problem? anyone? thank for the help in advance.