|The 100Gb was there when you went to sleep and missing when you woke up?|
"free space of data." There is no data that resides in free space...all data takes up space and free space has no data in it.
Anyway, what size is your hard drive. You give absolutely no idea of what you want us to work on. There's a good chance the 100Gb was never there and ain't missing after all.