If your additional hearts disappeared and no longer appears, it’s because the system is blocked. To restart the system, just write the following commands :
/setblock 1 75 47 air
/setblock 0 75 49 air
/setblock 1 75 50 air
/setblock 1 75 47 redstone_wire
/setblock 0 75 49 redstone_wire
/setblock 1 75 50 redstone_block
At your next death, your hearts will reappear.
Warning, Minecraft 1.8 has a bug in the Nether Dimension. If you enter in the Nether (Angband) dimension you will not see the additional hearts in your health bar. But d’ont worry it’s just a display glitch of this effect. The additionnal hearts system is still operational and your health will not move.
So for exemple : if you have all your life and a Zombie hurts you, you don’t will see the heart disappear directly because you have another heart that you can’t see due to the Minecarft 1.8 bug.