cant log into it

i have a windows 2000 pro loaded computer i can no longer log into wont take password. i have tried to log on as adminstrator. it tells me invaled password. i have tried everything i know to get in and cant. what can i do to get back in it is a stand alone computer not on a network.i just used it to learn 2000 on and it was working just fine till this happened to me if anyone can help i would be very thankfull.