You will need to contact your landlord or real estate agent immediately for assistance. Your real estate agent will need information about the problem and the type of hot water unit. Check all of the available information and give them these details when you contact them. Photos can be very handy too, which will help them see the issue (if easily visible).
If it's an emergency, such as a burst tank, or leaking water, you may be able to have it taken care of as an emergency repair. Make sure you know what is classified as an emergency repair in terms of your tenancy agreement or lease.
Refer to your local Residential Tenancy Authority for more information regarding tenants and landlord obligations in relation to repairs.