It depends. If you bought a fully-refundable ticket, the airline will issue you a refund.
In most cases, if you did not purchase a refundable ticket, the airline will issue you a ticket for future travel. Keep in mind, you need to let us know, BEFORE the flight because not showing up for a scheduled flight is considered a no-show and in that instance, you wouldn’t even qualify for a credit. For more Covid-19 related information, check out this post.