Recover accidentally deleted data from your Computer, digital camera, or flash drive

This is the most common issue, that you have deleted some important files from your system, that you didnt actually want to. 

Because the operating system doesn't immediately re-use space from deleted files, a file can be recovered or unerased right after it has gone and for a considerable time afterwards. The sooner you try to recover your deleted files, the greater your chances of success. The longer you leave it the less chance you have of getting the data back because eventually some or all of the space that held your lost data will be re-used by another file.

• The chances of recovery also depend on how full your computer's hard drive is. Windows tries to avoid re-using disk space that has recently been freed, to give recovery software a better chance of working. But the fuller a drive is, the sooner the free space is going to be used by other files.

If you have defragmented the hard drive since the file was deleted then this will severely harm the chances of a successful recovery. Current files will have been moved into the free space left by deleted files in order to reduce fragmentation, making it much less likely that undelete software will be able to find anything useful. 

How to recover deleted data:-
you can find many softwares of web claiming to recover data. Here is a list of some of the Freewares:

1. Recuva
2. Restoration
3. Undelete Plus
4. FreeUndelete
5. Photorec
6. Undelete360