Hi there LVandenberg,
I am so sorry about your hive! You should be able to see a cluster of bees on several of your top bars, or at least a decent number walking on their comb if everything was okay. If you are not seeing a cluster then you may have lost the majority of your bees. If you see a cluster their may still be hope. I wouldn't open your hive, because you don't want to let the heat escape. I would check back in about a week to confirm if your hive is going to survive or not.
If you are seeing a lot of dead bees on the bottom of your hive through the window it is likely that there was an attack by wasps that lead to the loss. Robbing from wasps can happen very quickly and they can destroy a hive in a very short amount of time. If there are only a few dead bees then this could have been a random absconding event. The remaining bees will likely keep doing their jobs until it becomes too cold. Without the other bees to keep the hive temperature consistent they will probably pass before winter.
I am sorry this happened to your hive! I have had hives that are good one day and then a day later something like this happens. Unfortunately it is sometimes the nature of keeping bees. I wouldn't get discourage and I would start looking at things to potentially improve upon next season (not to say that anything you did this time would have affected the out come). Please let us know if you have any other questions. You can always reach out here or to our customer service team at 877-325-2221 or by email firstname.lastname@example.org.