Do I have to do anything if my Medicare plan is cancelled?
Yes — doing nothing means you get moved or dropped automatically: your Medicare Advantage plan ends December 31, and if you make no choice you generally fall back to Original Medicare with no drug coverage unless you join a drug plan.
This is the single most expensive misunderstanding in Medicare. A cancelled plan does not quietly renew into something similar of your choosing.
What happens if you do nothing
Medicare's own reminder is blunt: if your plan is leaving Medicare, you must join a new Medicare drug plan to keep drug coverage. Prescription coverage does not follow you by itself.
The dates that matter
CMS lists Open Enrollment as October 15 to December 7, and Medicare Advantage Open Enrollment as January 1 to March 31. A plan termination also opens a Special Enrollment Period, so your decision window is not limited to December 7.
The one action to take
Choose deliberately: a new Medicare Advantage plan, or Original Medicare with a drug plan, and consider a Medicare Supplement while your guaranteed-issue right is active.