Sorry to the OP, who is probably kicking themselves harder over this than any of us could. Also sorry to say that at LGA, it is probably gone for good.

This is also an opportune time for a reminder to everyone that the 10000 foot chime is a good time to secure your stuff; the pilots are running through their checklist and you should be running through yours. Check the seat pocket, check around your seat and stash everything that belongs to you. I personally take care that the essentials -- my wallet, keys and cell phone -- are on my person in case of evacuation. Everything else should be stowed away on your person or in your carryon. It's also a good time to think about what you have in the overhead, especially with the winter months approaching and coats, etc. getting tossed up there. No system is perfect; I still lose things on trips, but what I do lose is minimal and easily replaceable.