Can a landlord/maintence person enter my apartment without notice?

What most likely happened is the water heater had a problem, and in that kind of emergency he can enter the property without notice. He should have tried to contact you or left a note. When he does get ahold of you make sure he is having it fixed and PRONTO. Otherwise not having hot water is technically an unfit property to live in. Especially if it's a rental. You can call your local housing authority and they will have more information for you.