Turn the phone off (it says you should be able to do this with the phone on - I don't trust many phone manufacturers). Remove the card.
Lightly rub the gold contacts on the card with a clean piece of linen (handkerchief, sheet [not polyester], etc.) Replace it. If it still isn't seen, you have one of two problems:
1. The SD slot is bad, and the phone will have to go to a repair shop to have it replaced. (Kind of rare.)
2. The card is bad. If you have a computer, put it into a USB adapter, then into a PC (or laptop) and see if it's readable. If not, try to contact Samsung or the seller for a refund. In the future, sty to stick to SanDisk. They invented the technology, they're one of the only 2 companies making the chips that go into them and ... in all the years I've been using their cards (and my smallest one was a 32
NB card, so that's a long time), I haven't had one fail yet. (I snapped one in half, but that was my problem.) If the card
does go bad, you go to
SanDisk microSD Tech Support, upper right, to the Live Chat, and talk to them. By the time the warranty runs out on one of their cards (5 years for the cheapest ones, lifetime for the more expensive ones, so it never runs out), the card will be too small about to care. (Who cares if a 256MB card fails now?) The company replaces them for free with no hassle. Samsung? They're under warranty, but whom do you contact? How difficult is it to get a card replaced? I've never heard from someone who has, so I don't know.