As far as I know, your contacts can possibly be stored in four places:
1) Your phone itself.
2) In a 'cloud' account, if you backed them up there. That account is accessed as Alisha C said by signing in using your user name and password. Then you should be able to import your contacts from your account.
3) Sometimes, on your phone's sim card, if you were able to export them to it. If so, you should be able to re-import your contacts from your sim card.
4) On an internal SD card, if you backed your contacts up by exporting a copy of them to the SD card.
Good luck!