If you had bought software at the store then the retail box would have had a product key on it. You need that to reactivate Office. Without that key you cannot license Office and would need to purchase it again. Personally I recommend that you store product key information somewhere electronically such that you can get back to it if you ever lose the box.
Alternatively you can purchase Office 365 subscription for a year cheaper than you can purchase a retail copy. Then it'll be tied to your MS account.