We bought our home about 3.5 yrs. ago.  We were unaware that we needed
to drain our water heater yearly.  now our pressure valve has erupted,
and NO water will be held in the tank.  Can we drain our tank & save
our water heater, or do we have to replace.  The water heater is still
doing a phenomenal job of heating and keeping the water hot.  Or is
there another method to save our water heater.  we are flat broke.

Not sure what you mean by "erupted". If it's a pressure relief valve
and it is on top (where they usually are) there should still be water
in the tank and you'd only have to replace the valve.  If it's a drain
valve on the side or bottom then replace the valve there. Since it's
winter and you used the word "erupted" I'm guessing that the water in
the tank may have froze and you very well may have to replace it.  On
the other the valve may have saved you and the tank may be okay. Try
and refill it with a new valve installed.