The answer depends on whether you are receiving advanced premium tax credits. For people receiving advanced premium tax credits, if a payment due date is missed, insurers must provide a 90-day grace period during which consumers can bring their premium payments up to date and avoid having their coverage terminated. However, the grace period only applies if an individual has paid at least one month’s premium.

Insurers that offer plans on  HealthCare.gov  are required to send prompt notices regarding payment delinquency (within 10 business days after they notice your missed payment).
If, by the end of the 90-day grace period, the amount owed for all outstanding premium payments is not paid in full, the insurer can terminate coverage.

In addition, during the first 30 days of the grace period, the insurer must continue to pay claims. However, after the first 30 days of the grace period, the insurer can hold off paying any health care claims for care received during the grace period, which means the enrollee may be responsible to cover any health care services they receive during the second and third months if they fail to catch up on the amounts they owe before the end of the grace period. Insurers are supposed to inform health care providers when someone’s claims are being held. This could mean that providers will not provide care until the premiums are paid up so that they know they will be paid. People not receiving advanced premium tax credits are expected to get a much shorter grace period; currently, the general practice is 31 days but it may vary in each state.

Note that insurers are no longer allowed to require people who owe back-due premiums from the past 12 months to repay the premium debt before they will renew or sell you new coverage for the year.