well ok then....
if you do not have your cd key for your copy of office, then you can not reinstall it. you'll need to buy a new copy of office if that is the case.
if you have the key still, then you need new install media which i'm not sure you can get from MS anymore. may be able to get it from other sources that archive disks like this. a google search should turn up a couple copies to download if MS does not have it anymore.