Question: Is it safe to de-fragment your disk when you have low disk space? What could be the consequences? What are the advantages and disadvantages of de-fragmenting. Is it possible to resume de-fragmentation of a disk after a few days? Can de-fragmentation of large disks be simplified as it takes lot of time to complete? Is it possible to lose data after de-fragmentation? Will it reclaim my Disk-space?

Answer: 
" Is it Safe to De-fragment your Disk when you have low disk space, what could be the consequences?"

The defragmenter will not complete the defrag efficiently if you have low free space if you use the Windows defragger. But you can run a third party automatic defragger which defrags under less than 5% free space and get the job done without any problems.

"What are the Advantages and Disadvantages of De-fragmenting.Is it Possible to resume de-fragmentation of a disk after a few days. can De-fragmentation of large disks be simplified as it takes lot of time to complete. Is it possible to lose data after de-fragmentation. Will it reclaim my Disk-space."

There are no disadvantages, advantages are that the performance will be smooth and data access fast.
With intelligent automatic defrag technology it's also possible to have a program defrag in the background without being resource intensive and will also defrag very fast as compared to the built-in program. Defrag doesn't create any new diskspace and there is no risk of data loss. Just ensure that you have a power backup for the PC.
As i understand..... Defrag has no risk.Yes it takes hell a lot of time.while you save a file datas are written here and there over the spinning media where it finds empty space.so when you read that particular file data has to retrieved from so many places as it was not written in sequence which makes the program consume more time. By de fragmenting system pick up the fragments and arrange in an order which makes the systems read files faster.Thats it.