Defragmenting does not free up space on your drive. In addition, most defragmenting software will either not work, or work extremely slowly on a hard drive that is mostly full.
1. rearrange the individual chunks of a file so that they are all together on your disk.
2. consolidate the free space on a drive so that it is in one lump.
3. re-organise the placement of system and user files on your hard drive to optimize access times and minimise the possibility of future fragmentation.
Not all defrag tools do all of the above; I currently use and recommend MyDefrag (formerly JKDefrag).
... i notced that i could not locate some of my important documents on my pc...
If files have gone missing after defragging then it is likely that they are lost. By its very nature defragging swaps data about all over the drive, which will defeat any data recovery efforts. Best head for your backups, if any; regular backups are a good idea generally but especially before defragging as a power cut during the process could cause massive data loss.
...but these unfound documents are still occuping spaces on my harddrive
How do you know this? What happens if you run a disk check on the drive?