Check meter, has it been turned off? Check with your neighbour/other residents. Perhaps there are temporary works within the complex. Call your water company to ensure it hasn't been turned off in the street.
No Hot Water?
Check the isolating switch (typically located at the switchboard and next to the hot water system, if installed) are both turned on.
Check the safety switch marked "hot water" or "HWS" at the switchboard.
Check the unit itself: If the unit is gas, check the pilot light has not gone out. If it has, can you re-light?
If the unit is instantaneous electric, check for an error code.
If the unit is outside, is there water coming out of it? If there is you will need a plumber. If it is electric and no water is coming out, you probably need an electrician.
Water Leaks?
If you have a dripping tap, it is most likely a washer and it will need to be replaced. If you do not know how to do this, don't try. If you do, the first step is to turn off the water at the mains supply before you proceed.
Blocked Drains or Sinks?
Usually caused by blockages and commonly food build up in kitchens or hair build up in bathrooms, blockages can be easily rectified with a plunger, Draino (available from supermarkets), or removing the hair and food that is causing the blockage.