In the past few weeks, hardcore ladder characters have been mysteriously dying. Blizzard has now announced their fix.

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For the past few weeks, characters playing on the Diablo Realms servers have been mysteriously dying - especially those on top of the hardcore ladders. Others have been stripped of money and equipment.

Here is Blizzard's official statement, but to sum up:
1) The bug which caused the problem has been fixed.
2) On Jan. 8, all hardcore characters who died between Dec. 19 and Jan. 1 will be revived.
3) Characters with missing equipment can be flagged to be included in the restoration