Are you sure that you've actually 'lost' the program. Perhaps you've just lost a desktop shortcut (or similar) to access it? Please post again, telling us what you mean by 'lost', to see if we can help you.
However nobody actually needs Microsoft Office anyway. There are several freebies which are every bit as good, with the most popular (by far) being OpenOffice:
https://www.openoffice.org/
(If you do decide to use OpenOffice, which I can thoroughly recommend myself, you'll probably find it useful to get it to save files in Microsoft formats by default. Simple instructions can be found here:
http://helpdeskgeek.com/office-tips/set-openof%EF%AC%81ce-org-to-save-in-microsoft-of%EF%AC%81ce-format-by-default/ )
The two other free office suites which are worth considering are LibreOffice
https://www.libreoffice.org/
and WPS Office (formerly Kingsoft Office):
http://www.kingsoftstore.com/software/kingsoft-office-freeware