I'm sorry about your dog too!
Since he didn't get up after his fall plus breathing problems, it is a possibility something in the flower garden stuck him (ie - a rose thorn or something similar). Many flowers/things(including insects) in gardens are very toxic to dogs and if something like that happened, it might have gotten under the dog's skin, out of sight.
Something like that can a) hit a muscle group and cause pain during motion (explaining some of the lethargy) and/or b) get into the dog's blood stream and cause an allergic reaction that becomes toxic in the blood.
You need to have a vet do a full blood work up on the dog, as quickly as possible and jot down every type of plant, flower and/or insect you notice in your garden as well as the brand names, lot numbers and phone numbers on the labels of any insecticide you use (if he fell on an insect with an ability to bite or sting, that insect had ingested the insecticide and if the stinger hit a blood vessel, your dog could be in need a blood transfusion)
If he is still not taking in fluids within 6-8 hrs since the ER vet removed the IV, consider it an ER. Dehydration will return and can be fatal if not treated.
They should do blood work on your dog and coupled with what you jot down about your garden, if you can't get to regular vet, an ER vet will notify the National Animal Poison Control Center if the results show a toxic reaction. This org will charge you for a phone call and you need the actual vet test results to verify if your dog has a poison/allergy reaction so it is better to let the vet call them. They are familiar with EVERYTHING that can cause reactions in animals, including andidote medicines. You dog might only need a medication vs a transfusion but there is no way of knowing unless you have a vet do the blood work.
Don't put it off. The longer you delay, the higher the risk, whatever it is, if caused by something in your flower garden, will reach the dog's heart. I'm surprised they released the dog if his breathing was not stable!
Get your dog back to a vet with a quick list and any labels you have from insectides. If you can't locate the bottle information, don't worry. Just tell the vet and if you saw the dog when he first fell, the general area (ie - backside, leg, chin, etc) that hit the ground first. They might be able to locate what would be a very small dot where a bite or stinger got the dog and if the latter, they might be able to remove it. If they can't, it will just pass through the dog's system but, only the blood work will be able to tell you if it's poisonous or an internal allergic reaction and let them treat the dog. There would be no other reason a fall would effect the dog's breathing and I'm sure they checked for any broken ribs.
It sounds like something more serious but completely treatable.
You can also use an eye dropper to give your dog water but, if he is not moving, he needs to go back to the vet.
Please post back how he is doing.