Os x startup problems

I wasn't really sure how to search for this question so i'm sorry if this has already been asked about and answered. My macbook battery was completely dead so when i unplugged it to go to another room it naturally died. when i plugged it back in it went to a login screen, asking me for a user and password. I had one before but i disabled it for a quicker startup. I tried using the same username and password from before but it's not working. any idea what's going on?