Search before posting, please. This is already stickied at the top of the bug forums:
In order to upgrade your gems, you can lead the mob to the next floor in the GR, then teleport to town and walk back to Urshi from the start of the rift.
You were robbed by a game bug, but this is the bargain you made when you started playing HC. HC Heroes will not be restored for ANY reason.
This is not the only game bug that will kill you in HC. Accept that you will lose heroes due to game bugs, or play SC.
RIP Sonya. Sorry for your loss.