United Airlines missed flight policy states that if a passenger fails to check in or board on time, the booking is usually marked as a no-show, which may result in losing the ticket value and cancellation of remaining flights on the itinerary. However, if you contact United immediately after missing the flight, they may help rebook you on the next available flight, sometimes with a fare difference or fee, depending on the ticket type.

https://www.flyfairtravels.com/blog/united-airlines-missed-flight