Have had this happen to myself twice.
Hero shot by Hel’s special, wait until debuff is gone, and they are still unable to gain mana.
The second time it happened it was with Rigard, waited for the debuff to disappear then hit a purple gem to use all the purple shields. This should have filled his mana, but didnt. He was subsequently hit by Chao’s special, his hitpoints went down, but his mana bar went UP.?
Possibly a glitch in the displayed mana not updating correctly after being hit by Hel.
A couple of other members of my alliance have noticed similar events