It is not possible to guarantee that UndeleteOnClick (or any file undelete utility) will be successful in recovering your deleted files. If you wish to recover an erased file now we strongly recommend that you do not read, write, change or delete any more files, or defragment your drive. Do not reboot or switch off your computer. Do not log off. Do not defragment the drive.
When Windows "deletes" a file, it is not physically removed. The file is still on disk, and may be recoverable, however, Windows makes the space occupied by the file available for writing. If you create a new file, or save a changed file, it may write over the 'deleted' file's space, thus destroying it permanently.
Be aware that because of the reasons stated above, a file may appear to be recoverable but may turn out to be corrupted or unreadable. This is not a shortfall in the performance of UndeleteOnClick but a result of the way the Windows operating system handles deleted files.