Short answer: You can keep seeing the provider at out-of-network rates, switch to an in-network provider, or, in many cases, request continuity of care to finish active treatment at in-network cost for a limited time, especially during pregnancy or a serious illness.
Provider networks change during the year. If your doctor or hospital leaves the network, your plan will still cover care from remaining in-network providers, but seeing the departed provider usually shifts to out-of-network cost-sharing, which is higher, and may not count the same way toward your limits.
Many plans and several states offer transition-of-care (also called continuity-of-care) protections. These let patients in the middle of active treatment, such as pregnancy, chemotherapy, or recovery from surgery, continue with the same provider at in-network rates for a defined period (often 60 to 90 days, or through the end of a pregnancy). You generally have to apply and show that you are in an active course of treatment.
If your provider leaves, call the carrier promptly to ask about continuity-of-care rules, get any approval in writing, and confirm how long the protection lasts. Otherwise, ask for help finding a comparable in-network provider.