The drooling is due to nausea, this could also be why the mouth was open. The "slumping" is likely due to lethargy or possibly trauma. It is hard to guess from the description the exact cause. Trauma, dietary indiscretion, toxicity, infectious diseases, and metabolic diseases are all possible causes. It would be best to have your cat examined by a veterinarian to diagnose the problem and start supportive care for the nausea and lethargy.
Drooling in cats can be normal due to food smell, excitement, happiness, and hot and humid environment. Excessive drooling can be due to teeth and gum diseases, foreign body trapped in teeth, mouth ulcers, ingestion of medicines and flea sprays, ingestion of small lizards, infection of salivary gland, liver disorders, feline herpes and feline calcivirus.
Proper diagnosis requires thorough examination or oral cavity and some lab works. Sometimes, anesthesia is used for oral cavity examination. SO, take your cat to vet.
This could be the result of a stroke, seizure, or the ingestion of a potentially dangerous or foreign substance. It would be highly recommended that you bring him/her to a vet ASAP. I wish your cat the best of luck.
Sorry but first thought was a possible broken jaw, although it's more likely to be an infected tooth or injured tongue. Without more details it's hard to guess. If this is not normal for him and if he's not eating/drinking as usual, and/or appears to be in discomfort, please take him to the vets asap. It may be something other than his mouth that's causing him to do this. You don't want the little guy to be in pain.
