defrag problem with lack of space

The process of defragmenting is two-fold. First, you need to tell Windows to "analyze" the disk, in which case it will scan the disk and find all the fragmented pieces, if any are there - you will need to have at least 20 percent free space to do so (it needs the extra space for putting back the fragments later). Once analyzing is finished (which doesn't take long), you can hit the "defragment" button. It's a process which can take several hours so the recommendation here is to defrag at night while you're sleeping. This was an answer to someone else but what happens if you run the defrag and didnt have enough space and now computer lost stuff?