Had a shower yesterday with no problem, and this morning the hot water tap had little to no pressure and no hot water. We turned the lever on the hot water tank on and off, and also on the main water system. We now have hot water. What happened and how can I fix this problem before it happens again.

When is the last time you flushed your water heater?

I didn't know you had to, and do not know how to "flush the system". Are you able to explain, please.

See that hose bib at the bottom of the tank? Connect a hose to it, take the other end out side or stick in in a drain. Open hose bib and let water run until.l water is clear. Sediment and mineral deposits collect in the bottom of the tank and can clog dip tube. Some times the hose bib will not reseal properly because of sediment. If that happens get a cap and put on hose bib.

The Widget Maker gave you a excellent answer. Let me flesh out this flushing thingy a bit more.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back together the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps and thank you for rating my answer, Tom