My cat is Lethargic, mouth breathing and won't eat

I woke up this morning to find my cat in a bit of distress. She is breathing with her mouth open and her tongue out a little. I also tried to give her canned food and she wouldn't eat. I brought her to the emergency vet, they took her temperature and vitals and X-rays and she wasn't showing anything significant. I have now brought her home and she still won't eat and is very lethargic.
I have tried to syringe her a little watered down cat food, but she kind of gags on it.
Is there anything else I can do for her? What can I ask the vet?

Open mouth breathing is indicative of respiratory distress (at least, I don't know of any other reason for it). I can't say the cause, but when I've seen it it's been related to fluid on the lungs and/or lung inflammation (ie asthma or complications from heart disease). It could also be caused by an allergic reaction or, obviously, some kind of obstruction in the airway.

I'm very surprised nothing showed up in the tests. I would have expected any mixture of the following treatments (depending on cause) - steroids, diuretics, bronchodialtors, oxygen, antihistamines. I'm very puzzled as to why they sent you home without treatment (which is what you seem to imply).

All I can suggest is go back to the vet (or at least call them) to clarify what is causing the problem and give your cat some relief. To my mind, it's not acceptable to let open mouth breathing continue untreated, and I'm surprised you've been left in this position.

I believe they gave her some puffer at the hospital, but I wasn't sent home with anything. If she shows no improvement, I will call and go back tomorrow if not later tonight as the clinic I took her to is open 24 hours. She has been sleeping quite soundly since we got home but it has been several hours and she still hasn't eaten anything or used her litter box despite the x-ray showing she should. 

Certainly could be breathing issue that doesn't show on X-ray, and trouble breathing will also make a cat not eat. Ask what else could be causing the breathing issue suddenly that wouldn't show on the X-ray. Has her breathing improved at all since the vet and whatever treatment they gave there? If not you should definitely call at least, maybe they can suggest something and/or prescribe something without her having to go back (just you to pick up med if necessary). If she seems to be breathing better the not eating At this point could be just from the stress of the day, but I wouldn't chance just waiting around to see unless you are quite sure. Unless her breathing is improved she's not going to eat.

When I first brought her this morning she was panting. There is no panting now and her breathing is more even but her mouth is still open with her tongue just peeking out. Gertie spent quite a bit of time at the emergency vet today and was quite stressed while we were there. I am going to monitor her and see if when she wakes up she will drink anything, even some water would make me feel better. 

You can try steaming her in the bathroom.  Get the bathroom good and steamy, then sit in there with her until the steam dissipates.  This will help open up the nasal passages.  Keep trying to feed her via syringe.  Warm up the food just a bit to enhance the aroma.

I hope Gertie's doing better now. As stephanietx says, a steamy room can really help with breathing issues. It's not that surprising that she's not wanting to eat on her own - she may well not have the breath to eat. Go really slowly with syringe feeding to help with the gagging, and aim slightly sideways instead of at the back of the throat. Gagging from a syringe usually occurs if the cat just can't swallow because it's going in too fast or at the wrong angle - meaning the cat physically can't swallow fast enough.

I hope she's doing better. It's horrible when they're fighting for breath - it can end up making you feel so helpless.

Gertie and I had a late night at the emergency vet, but I took her back in - it seems earlier I had noticed the odd behaviour before she had fully developed symptoms as the vet has determined that she has a salivary mucocele that is causing swelling. She has started eating soft food that has a little water added to make it easier to lap up. Her breathing is better and we have pain meds and some Royal Canin Recovery. She is much more active today and much more interested in her surroundings. The vet said it more looks like mouth breathing than actually being mouth breathing due to the swelling. She will see her regular vet tomorrow.

Good job!!  Glad she's feeling a little bit better and is eating on her own.  Hope she continues to improve.

Really good that she's doing better. Hope she continues to improve