If you're absent for too long, I assume that an owl or teacher comes to check in on you. As for alternative means of travel, if it's an emergency, you may hail the Knight Bus and ask to be taken to Hogwarts. It will not be a pleasant ride by any means, but you'll be there quickly.

According to Wizarding World lore, the Hogwarts Express is the only "authorized" way for students to reach the school at the start of the term. If a student misses the 11:00 AM departure from Platform 9¾, there is no official "backup" transport provided by the Ministry of Magic. In the most famous instance, Harry and Ron famously "missed" the barrier and took Arthur Weasley's enchanted Ford Anglia, which led to a crash and significant disciplinary action. Generally, if a student were to miss the train, their parents would likely have to contact the school via Owl Post to arrange an alternative method, such as a Portkey or Floo Powder connection, though this would likely result in a loss of house points or detention. The train is strictly required to ensure all students arrive simultaneously for the Sorting Ceremony and the start-of-term feast, maintaining the school's high level of secrecy and organization.