Alma struggled with substance abuse in The Queen’s Gambit. In addition to taking pills to stabilize her mood — likely because of her unresolved trauma stemming from her biological child’s death, as well as the general anxiety caused by her emotionally unavailable and cruel husband — Alma also drank. Although she isn’t shown drinking to excess to the same degree Beth would later in the miniseries, Alma did drink regularly and had done so for a long time. She also was shown smoking cigarettes regularly, even in bed.
If Beth was correct that Alma had hepatitis, she could have died from cirrhosis (scarring) of the liver. Another possibility was that she experienced an aneurysm as a complication from the hepatitis or some other undiagnosed condition related to her liver, such as cancer. This would explain why she didn’t have more obvious symptoms prior to her death, such as jaundice, and why she was seemingly fine the night before her death.