What were the roots like when you repotted it? Although it is very hairy, the body of the plant is looking very thin and makes me wonder whether it is still alive or not, thus my question about the roots.

Please tell us what you mean by "looking a little sad." The cactus doesn't literally have a frowny face so I'm guessing there is something about how the plant looks that has you concerned. I'd prefer you tell us your concerns rather than have us guess.

My concern is that the skin of the entire side of your cactus looks white or yellow beneath the spines and hairs. Perhaps you put it out in full sunlight without gradually increasing the amount of sun.

I'd guess the yellow stripe is preceding rot. I hope I'm wrong.

Update from today. The body looks very yellow and it hardly staying upright. the roots were alright but it had been in a pot with poor soil and probably no water for at least a year.

what should I do? I have it in full sunlight thinking that when I transplant and watered it maybe it was too much but I’m not sure now. This all happened a week ago

Is there anything I could do to rescue it? Should I cut it or do something?

In your last pic it looks like it has been cooked by too much heat somehow. Cutting the plant won't help that. Where are you located in the world? We don't need an exact address but a town name might help us get an idea of what your climate is like. On what side of the building did you place the plant outside?

I’m located in Las Vegas en it is on the north side of the house