If you have confirmed your snake is dehydrated, then correcting this is your first priority. This is usually not too difficult to do. Please note that the snake should NOT be fed until it is properly hydrated. it is actually unlikely to accept a meal if offered, as dehydration is one of the factors that can cause snakes to go off feeding.
A snake that is dehydrated has lost not only a significant amount of water from its body, but also a significant amount of the electrolytes that are an integral part of that fluid. It is therefore essential in the rehydration process to not only replace the lost water, but also the lost electrolytes. The simplest way to do this is placing the snake in a tepid bath (28 – 30 degrees C) containing electrolytes in the water Snakes will normally drink some of the bath water. As well, fluid that enters the cloaca can be absorbed through the mucous membranes there. This should be done using a shallow container with a clip-lock lid that has some holes in it for fresh air. The bath water should about 1 – 2 cm deep (not entirely covering the body), and consist of 1 part water to 1 part electrolyte replacement drink such as Staminade, Gatorade, Powerade or similar. Place the snake in the bath, fasten the lid and keep an eye on it for the next 20 – 30 minutes, before removing and gently towelling dry the animal. If need be, this process can be repeated after several hours break. If this technique does not work does not work after say three treatments, then you are best advised to visit an experienced reptile vet. I personally would not recommend beginners attempting to syringe fluid into the animal via its mouth. You need to know how to do so and how much to use, and first attempts should only be performed under experienced supervision.
Once that is attended to, it is important to try and work out how and why your snake got dehydrated in the first place. This is so it not happen again. If you would like assistance with that, just say so. You may feel you are out of your depth at the moment, but it has just been an unusually steep learning curve due to circumstances. Once you are past this hiccup, you will get your confidence back and be all the wiser for the experience. Just remember the old saying: “A calm sea never made a skilled sailor”.