If it was accidentally deleted you can find it in your trash can in your OS. If you have trash can disable or you already deleted it from there you can try some recovery program like Recuva. It scans your HDD and finds deleted files but only on HDD that use magnetic discs inside it. This wont work for Flash memory like USB nor for SSD HDD. If problem isnt in your HDD you may have forgotten to save the file or PC may have shut down while you were working with unsaved file. In this case there is no way to recover the file. There may be a chance that program like MS Word or Excel autosaved your document but if not, its lost.