Having the bios able to see the disk, but a live boot can't, makes no real sense to me. If a partition was messed up I'd get that, but to not even see there is a disk to partition, doesn't feel right. I know it's probably a dumb question, but you didn't happen to be messing around in BIOS settings or something right? Is it possible you changed some settings a while ago but haven't rebooted in a while, and this issue was waiting for you this whole time?

If you don't have any other slots on the mother board to try the disk in, you could buy an external adapter for whatever kind of disk you have, which would allow you to use this thing as a USB drive. That should at least allow the live boots to see it.

Also also, is it possible you have two disks, and grub is on one and your data is on the other? Again, kinda weird question, but it's a kinda weird situation...

This might not be your issue, but it might be useful to know: if you connect two disks with the same GPT UUID, they might start misbehaving. Perhaps GRUB might not care, but Linux does. Did you ever clone a disk or partition without changing the UUID?

Like others said, sounds like hardware. But before you toss the drive, you said SSD, if it’s an ssd try different cables connecting to different ports. If it’s nvme try reseating it at least, moving it to another port if you have a 2nd. Just saying that sometimes ports and cables fail, so make sure you rule those out before losing hope.

Also possible it’s a bios thing, like maybe the port itself on got disabled in bios or controller got switched to raid mode.

CachyOS is very btrfs savvy. I would try booting from a CachyOS ISO.

I'm a btrfs noob, so I'm skipping any tools that may fix the disk, try those first.

If the media isn't dying, try a file carving tool like photorec. Idk if testdisk supports btrfs but it's worth a try, it's my go-to for undelete and finding lost partitions.

Edit: missed most of your post. First you need to check the kernel logs using the dmesg command. Look for errors that may explain why the disk doesn't show up, especially if it lists scsi or sata in the message.

Edit again: you may want to check the disk's self reported heath using "smart" data. Many bios menus show this info, and there are programs to get it on Linux. If there are too many read or write errors, you need to decide how important the data is. Professional recovery can probably get all of your data if you stop using the drive now and send it in. DIY recovery using a file carving tool would work best if you have another disk to make an image of the failing one with. ddrescue would be the tool for the job to create the image. If you don't have another disk large enough, and the files aren't super important, you can run file carving on the failing disk directly, but the more you use it the greater chance the disk will corrupt more data.

What you describe sounds like a hardware failure (one where btrfs plays no specific role).

If that's indeed the case, you can only bring the drive to a data recovery service and see what they say (if it's a spinning disk, they'll probably recover the data for an exorbitant fee, if it's an SSD idk).

PS: this is unlikely to work, but.. you can try cleaning the drive's contacts to see if it makes any difference, and also try moving the drive to a different connector (or use it on another computer)

If they don't see the drive at all it's fucked. If they see the drive but not partition I normally use testdisk to try and recover the partitions, photorec to look for files if that doesn't work and it's not encrypted.

If it isn't visible at all then you are looking at drive repair, if you are asking this question that probably isn't something to attempt. Try another set of cables and port just in case that is the issue though.