If you become disabled and are enrolled in Medicare, then you have to stop making contributions starting in the beginning of the month in which you enrolled for Medicare. Your HSA account is still active and can continue paying for qualifying medical expenses (including payments for Medicare Part A and Part B). Since you are disabled, any distribution you make for non-qualified medical expenses may not be subject to the 20% penalty and will continue to be tax-free.
Source: IRC Sec. 223(f)(4)(B)