The rules of travel to Europe have changed. Starting from 2024, US citizens (along with some 1.4 billion people from over 60 visa-exempt countries) are required to have a travel authorization (called ETIAS) to enter European countries that are part of the Schengen Area. Hungary is part of this Schengen Area. In order to obtain such a travel authorization, you need to submit an application via the European Travel Information and Authorisation System.

More information is here.