There is no hot water in the apartment, what should I do?

Turn on all faucets and showers and let the water run for 5 minutes. If it doesn't return, the problem is likely general and should be addressed by the building. Check the type of system; it might be gas or a boiler and could have an unregulated temperature. Usually, the systems are located in the laundry room or near the kitchen of the apartment, if there is one.