Answer:
There are numerous ways to get a cat out from under the bed. If the cat likes food, try shaking his/her food bowl to get their attention. Another way is his/her favorite toy. Coax it with the toy, make the cat chase the toy. If all else fails, crawl under the bed and retrieve the cat.
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Did the cat go under the bed because it was simply curious, or wanting to be alone, or scared? If one of the first two, trying to coax it out will probably make it scared if it wasn't already. If it was scared to begin with, you don't want to make a fuss because this will convince it there's definitely something to be scared of. The last thing you want is a frightened cat in a place you can't get it out of.
Unless you have an excellent and urgent reason to get the cat out from under the bed, the best approach is to leave it there, go out of the room, and find something else to do. The cat will come out of its own accord when it's ready. When it does come out, pet it and reassure it, if it wants you to.
And in future, keep the bedroom door closed if possible, in case it's decided under the bed is a good place to be.
If you really, truly must get the cat out immediately, use a spray bottle to spray plain cool water at it and then stand back. If you don't have a spray bottle, improvise some other way to splash water at it; any small plastic bottle you can squeeze will do.
If the cat doesn't come out in a great hurry once the water hits it, your only option is to get someone to lift the bed at one end while someone else grabs the cat. Or, if you're able, lift the bed yourself; the cat will probably go away when it sees it's lost its dark, special place.
Hopefully it isn't about to have kittens, another reason cats hide in dark, quiet places.
If you suspect this is the case, the water treatment or lifting the bed should be used, but first organize a cupboard or somewhere else small and dark, just slightly open, and lined with newspaper, to put her in. And then leave her alone, but, again, shut the bedroom door and any other places you don't want her to go.