As hot water heaters age, they break down. One of the most common maintenance issues with water heaters is corrosion in the tank. Failure to properly maintain a water heater can lead to the bottom of the tank rusting out. There is no repair for this issue; your only option is to replace the entire unit.
Other items on the water heater, like a blow-off valve, can easily be replaced. Find a trusted Water Heater Repair Specialist, like Atkins Plumbing, to inspect your water heater and discuss your options. Often the decision is a matter of investment and the how long you can extend the life of your water heater.