I'm sorry to hear this.
I need to ask you a lot of questions, I'm afraid.
How long has the tank been running? If it is new, did you cycle the filter before getting any fish?
Have you bought any new fish in the last couple of weeks?
Have you sprayed anything in the same room as the tank, done any decorating, used any air fresheners, or anything like that?
How often, and how big were your water changes?
Do you know the pH of the tank water just before or after the fish had died? If you haven't emptied the tank yet, test the pH or take a sample now ready to take to a shop when they open for them to test it.
Do you know how hard your water is? Does your kettle/shower head fur up - that's hard water if they do. Or look on your water supplier's website.
Do you know your KH? KH is a type of hardness that buffers the water against pH changes. It is not the same as the hardness that furs up the kettle, but they tend to be low or high together. If you have soft water you are likely to have low KH.
If the tank was new and you didn't do a fishless cycle, they could have died from ammonia poisoning. Neons are the most susceptible of your fish, and when they died their bodies would have decomposed to make even more ammonia.
If you have bought new fish, they could have brought in a disease. There are some that could have wiped out a whole tank that fast.
If you have used air fresheners etc, they can get into the water and poison the fish.
The community creator puts you at 175% stocked (though I did have to substitute 5 smaller panda cories as peppered cories aren't in the database). This amount of fish makes a lot of waste, especially the two bristlenoses. Lots of waste means lots of ammonia which the filter turns into huge amounts of nitrate.
Around 12 years ago, I got up one morning to find every fish in the tank dead. I had no idea what happened back than as we didn't have internet access. But with hindsight, I think I know now what happened. I will explain what I suspect happened to my tank, see if it fits with yours.
I have softish water that has a low KH. My tank was overstocked so lots of nitrate, which is acidic and pushes the pH down. I didn't do big enough or often enough water changes which allowed the nitrate to build up and my low KH to get used up. I didn't have a test kit so this is my best guess - the pH dropped very low very quickly and that killed my fish.
What to do.
If the tank was new, empty the water, refill and do a
fishless cycle before getting any more fish.
If it was a disease brought in by new fish, you will need to remove every trace of it before restocking the tank. Unfortunately that means sterilising the tank with bleach, then re-cycling the filter. I do know that is not a pleasant prospect; it is what I had to do when my betta got lymphocystis. It was 6 weeks before I could get another betta.
If it was something in the air, you need to empty all the water and refill the tank. The filter bacteria should be unaffected but it is worth getting a bottle of ammonia solution and adding some to check the filter can process it before getting more fish - and stop spraying, using air fresheners, whatever it was. Use the fishless cycling method for the amount of ammonia to use.
That leaves possibility number 4.
The first thing to do is test the tank water's pH to see if it has dropped very low. A shop will be able to test this if you don't have your own tester. Take a sample from the tank now ready to test when the shops open. And you need to know your KH - again a shop will test for you but for this take a sample of tap water as you need to know how much buffering capacity your tap water has. And it's KH you need to know not GH so make that clear to the shop.
If it does turn out that you have low KH in your tapwater and your tank's pH did drop very low, you can restock the tank, with precautions to avoid this happening again. First, change all the water. That will reset your KH and pH back to tapwater levels. Then you can get more fish fairly quickly before the filter bacteria go dormant though lack of food. But this time don't get as many fish! Use the community creator on this site so you know when the tank is full. Click on fish profiles in the menu at the top, click on any fish then scroll down to the bottom of the page. Just enter the tank details and add fish from the profiles. If you want to save your tank, you do have to register and log in separately from the forum.
I would not get more bristlenoses for this tank as it isn't really big enough and they do produce a huge amount of waste for their size. And if you want cories, choose 6 of one of the dwarf species. The most common one in shops is pygmy cories but that isn't in the fish profiles. Use salt & pepper cories (which aren't the same as your peppered cories) in the CC.
Sorry for the essay, but I do know what it is like to lose a whole tank of fish. See which of the possibilities is the most likely for you - and any others that other members can think of.