On the tank there is a release valve. You will need to lift this valve and water will now start to fill the tank. The re-filling can take a few minutes, but should take no longer than that. In some cases there will be a small drip from the valve when the tank has been filled. Allow up to 24 hours for the tank to re-heat.
It may also be an idea to check with your electricity provider if your electricity account is on a night rate (offpeak) which means that the system only heats water overnight and not during the day. This is suitable for a small family however if you are finding you regularly do not have enough hot water you may need to contact your electricity provider to have this feature turned off. Please be aware this will increase your electricity costs will then be heating water 24hrs a day.
If the hot water system is a gas system, the pilot light may have gone out and you will need to re-light the hot water system. If you are new to the house this may not have yet been done, or on occasions if there is an air pocket in the supply or after a very windy day, the light may go out. The instructions on how to re-light the system will in many cases be on the outside of the system.
If you have gas bottles, the bottles may be empty. If the previous tenants did not use all of their gas you will still have a supply until they run out. It is your responsibility to have the gas bottles re-filled regularly. Failure to light the pilot light or if the gas supply has simply run out this could result in the service fee being on-charged to you.