If you reserved a room the night prior to arrival and did NOT check in and did NOT inform them that you would be arriving first thing in the morning, then I am sure they considered you a no-show and in that case, they would have every right to sell your room. Some hotels even would reserve the right to charge you the night as a penalty.
Point is - if you are not going to arrive the night of your reservation, then I don't understand how you could expect them to know you still planned on arriving at all? Sounds like your travel agency really dropped the ball and that is who you should have issue with - not the property.