My moon cactus is deteriorating. Please help.

Hello,
I bought a moon cactus few months ago. When I got it, its green part was thick & fleshy, but now suddenly it has become very thin & it feels like there is absolutely nothing in it.
Secondly, I replanted it a few days ago & I observed that, its roots are too small. Are their roots actually so small or is anything wrong with my cactus?
Thirdly, its lower part has become yellowish.
Can someone please tell me what is wrong with it and how can I revive it?
Thankyou in advance.
I bought a moon cactus few months ago. When I got it, its green part was thick & fleshy, but now suddenly it has become very thin & it feels like there is absolutely nothing in it.
Secondly, I replanted it a few days ago & I observed that, its roots are too small. Are their roots actually so small or is anything wrong with my cactus?
Thirdly, its lower part has become yellowish.
Can someone please tell me what is wrong with it and how can I revive it?
Thankyou in advance.

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I'm posting pictures of its roots & the yellow part.
Put the plant somewhere where it gets plenty of light.
I really feel bad for buying it.
I think I should have researched a bit before buying it.