Chicken Stock - Absorbed all water?

You don't mention covering your pot, so I'm assuming you left it uncovered for the duration. If that's the case, you boiled all your liquid off. Next time you can try using a lid, that should slow the evaporation of water from your pot and leave you with some stock still in the pot. Even if the pot was covered, though, "a couple of hours" is a pretty long time for something that only had enough water to cover-- no lid seals perfectly. Some evaporation is always to be expected, so you probably needed a little more water to start, or to have topped off the pot at some point.

You may be able to reuse your chicken pieces and vegetables, but I wouldn't. You mention using chicken breasts. If your aim is to produce a flavorful stock, this is the wrong part of the chicken to use.

Meat is typically used in broths, which can be flavorful, but chicken breasts are notoriously bland even when cooked whole. Stocks are typically made with bones and very little meat. Rendering the flavor and gelatin from the bones gives a rich stock using much cheaper cuts (or even scraps, if you have a recently roasted bird). Price may or may not be a factor for you, but the result is usually far superior when using an appropriate ratio of bones:water.

In your case your chicken breast is like so dry and overcooked at this point that there's not really any flavor left to impart to a stock/broth. You may have slightly better luck with the plant matter, but I doubt it.

Not absorbed (what would absorb it?)....evaporated. Even covered there was probably room for water vapor to escape. If you've stored them properly since...or this just happened. Cover with water, and simmer for 45 minutes. Strain. Use the stock. It won't be ideal, but you can avoid wasting the original ingredients.

Since your confusion seems to come from not knowing where the sweet spot lies between too much and too little water: I looked up in Ruhlman's book "Ratio", and he recommends 3 parts water to 2 parts bones. On top of that, add mirepoix such that it makes no more than 20% of the total ingredients, which comes to a total ratio of roughly 3:2:1.

With that ratio, the problem can no longer be the water. If you again get too much evaporation, your problem is wrong temperature (too high) or wrong vessel (too high ratio of area to height). If you get a flavorless stock, you are using the wrong parts (white meat instead of bones), or low quality ingredients.