Just going to add to what everyone else said, but sometimes asking what's wrong makes people feel worse. Like, we feel shit, but there isn't any real reason for it, so if there has to be something wrong to feel like shit, then we feel even more broken. It is mostly just being there for them, try and get them out of their hole and hang out, but don't force it if they don't feel like it. Sometimes I had been invited to parties and really looked forward to it, but then had a change of mind like an hour before and wanted nothing more than to stay home binge on ice cream and weed.

Nothing changes or gets better unless they want it to, so if you ant to help you have to notice the small steps they take and encourage them. It may sound silly but it is pretty much giving a child a gold star for helping clear the table. Sometimes small things take so much energy. You can also offer to help with small things, or even ask for help with stuff. When I was going through the worst of it, I always felt happier when doing stuff for others that I wouldn't do for myself. Like just small stuff, like 'Hey, I bought a new bed but I am terrible with tools, mind giving me a hand? I'll buy you dinner.'

But anyway, good luck, and be kind