Let’s assume that you don’t have a physical device failure (which is a different set of tools – mount -odegraded, btrfs dev del missing).
First thing to do is to take a btrfs-image -c9 -t4 of the filesystem, and keep a copy of the output to show josef. :)
Then start with -orecovery and -oro,recovery for pretty much anything.
If those fail, then look in dmesg for errors relating to the log tree – if that’s corrupt and can’t be read (or causes a crash), use btrfs-zero-log.
If there’s problems with the chunk tree – the only one I’ve seen recently was reporting something like “can’t map address” – then chunk-recover may be of use.
After that, btrfsck is probably the next thing to try. If options -s1, -s2, -s3 have any success, then btrfs-select-super will help by replacing the superblock with one that works. If that’s not going to be useful, fall back to btrfsck –repair.
Finally, btrfsck –repair –init-extent-tree may be necessary if there’s a damaged extent tree. Finally, if you’ve got corruption in the checksums, there’s –init-csum-tree.