Information is scattered around the site, I don't think I can close this as a duplicate of any 1 Question.
We have several good Questions already, dealing with aspects of an administrator leaving:
Additionally, nobody should know the "main" administrator password. You should have at least two people use a random password generator, each writes down their half, types it in, then seal the two halves in an envelope (preferably in the company safe). Nobody knows the whole password this way.
It sounds like you probably aren't the system admin, but have been drafter on account of that person quitting (or whatever). The next person should have a read through The Practice of System and Network Administration. It's as close as it comes to the System Administrator's bible, which covers this sort of situation and gives insight into mitigating the problems you're having.