Sevin is nasty stuff, harmful to everything in the environment, including plants, though it is supposed to be a pesticide that can be used on plants We have other people write in to say their garden was killed when they sprayed with Sevin.
Here's an article about what Sevin does in the environment
Scrolling about half way down it says this:
Plants: While insecticides are not usually assumed to have adverse effects on plants, carbaryl's use as a plant growth regulator (chemical thinning agent) in apples95,96 makes effects on other plants unsurprising. The following four types of effects of carbaryl have been documented in crop plants:
* Effects on reproduction. Examples include a decrease in germination success in wheat97 and decreased germination and an increase in abnormal chromosomes in vetch.98
* Effects on growth. Examples include the inhibition of seedling growth in beans,99 disrupted cell division in onion,100 distorted growth in poinsettia,35 decreased growth in peas and vetch,19 and a decrease in the weight of bolls in cotton.101
* Effects on photosynthesis. Examples include reductions in the photosynthetic rate of pecan trees102 and young soybeans.103
* Effects on nitrogen fixation. Examples include reduced colonization and spore-formation of peanut mycorrhizae,104 interference with the nitrogen-fixing mechanisms of the soil bacteria Azobacter,105 decreased photosynthesis, growth, nitrogen-fixation, and survival of a nitrogen-fixing bacteria common in rice paddies,106,107 reduced growth of the nitrogen-fixing bacteria Rhizobium,108,19 and toxicity to another nitrogen-fixing bacteria.109
As to what to do, all you can do is spray off your plants well with clean water and flush water through the soil and then wait and see what happens. They may or may not recover, depending on a ton of factors like how much you used , what plants you have and how sensitive they are, how sunny it was at the time, etc.
Did you even give the Neem time to work? It is not a poison and does not kill on contact. It has to be ingested and then it works more like a hormone disrupter so the insect's system gets messed up and it stops eating. So after you spray Neem, it takes awhile before you see any results. What bugs were you trying to get rid of and how bad a problem were you having?