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Was this started in water or soil? The water method is very popular, but it does leave the seedling very prone to infections. Fortunately, this can be saved. It is important to plant in container right away, but firstyou will want to treat the infection.

The best, and easiest way to do this with a seedling is to mix 1/4 of a cup of peroxide to 3/4 of a cup of water. pour this over the top of the young seedling slowly until it drips down the roots. This will kill off the infection and prepare it to plant.

This article will help you to grow an avocado: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

This article will help if you are going to grow this in container, since you are outside of their native zone: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

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Unfortunately, this could happen for very many reasons. This can happen for just about any kind of stress to the tree, including pests, over and under-watering, too much ultraviolet light, and many more. Here is an article that will give you a better idea of what you are dealing with: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm

Avocado is a tropical plant and will not survive where temps go below freezing. Here is a map with hardiness zones: https://www.gardeningknowhow.com/planting-zones/usda-planting-zone-map.htm
You can also do an internet search "hardiness zone by zip code".
One alternative is to grow avocado as a houseplant. https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

It is possible that if you purchased the seeds, they are already suitable for a fruiting plant. Still it takes two to produce fruits. Male and female flowers bloom on separate trees.

Now, when you graft, you can be sure that the fruit you get will be just like the branch you grafted. Alternatively you can graft a male branch on a female tree so it can be self fertile. You will just have to ensure any grafted branches receive much attention so they do not dampen off.

Here is an article that will help:https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

I will tell you my method, which works every time. The first step is to Slightly moisten a paper towel and wrap the seed in it. Place this in a ziplock bag, and put it in a dark drawer. Check it every couple of days until you see a root coming out, then plant it in the soil. This method is much easier than many others, and has worked for me %100 of the time.