just answering the question you asked. That is what would have happened if you didn't call to confirm. They need confirmation from the hotel and if they don't get it they cancel the booking. Same with flights.
Check your passport. Some embassies glue a visa in your passport and then stamp it with a DENIED-text. If they did that, you'll need a new passport.
If you don't have anything new in your passport, and you haven't been in Thailand long-term before going to Hanoi, you can enter Thailand on a 45 day visa exempt, which should be enough for your 1 month trip.