One of my stones fell out!

Read your terms of purchase and definitely contact the dealer - they should make it right or cover repair costs if it was defective or poorly set. And if you got the ring insured, that should cover the repairs.

You should be able to. My ring was from online and I had it sized at Helzberg. They assured me sizing it would make the stones fall out. One of the pave stones fell out a couple days after I got it back. They replaced it free of charge. You'll probably need to pay since it isn't their fault, but they can probably do it. But, contact the vendor for your ring first and see what they cover.

They should replace the stone for you- otherwise If it's insured you should be covered

I would contact the original seller of the ring and see if there's anything they can do. If not, then you might be able to bring it into one of those stores and see if they can replace the stone. You will most likely have to pay for it though.

Depends on where you bought it. I bought mine at zales. The diamonds came with a warranty and we purchased additional ins for metal work. With them as long as you keep up on the annual inspections the warranties would be valid.