Hot water tank runs out of hot water
First do as @Tyson has suggested and check the heating elements. If the elements are okay, the cold water inlet has a "dip tube" to insure that the new water introduced into the water heater when hot water is being used may have broken or worn out allowing the cold water to enter the top of the tank instead of going to the bottom of the tank. This would cause the tank to presumably run out of water quickly. To check this the inlet piping needs to be removed and the dip tube taken out and inspected. If you are paying a plumber to do this work, it may be better to just install a new hot water tank. I would compare the cost of repair to the cost of replacement to see which is the better value.