LawOfficer.com says the cause of death is not yet known, at least not publicly known.[1] It seems that he died of a "medical condition", and not having his skull crushed as at least one person on Twitter has stated.
The same website has, as the top article:
"Capitol rioter filmed hitting officer in head with fire extinguisher"
Which quotes: "Two law enforcement sources told the Associated Press that Officer Brian Sicknick, 42, was hit in the head with a fire extinguisher, according to The Post. He later died at a hospital Thursday night"
1. https://www.lawofficer.com/family-of-fallen-capitol-police-o...