connecting keyboard to mac mini

I bought a new Mac earlier this year. My mac mini has sat idle these past 8 months while I was busy with other things. Now I'd like to sell it. But I cannot sign into it. I replaced the batteries in both the mouse and the keyboard. The mouse works, but I cannot enter my password to log in. Meaning when I type the password on the keyboard nothing gets entered in password field. i even replaced the patties a second time. How do I get the keyboard to work?