Mac storage basics · When the disk is full
What happens if my Mac completely runs out of disk space?
Short answer: Apps can't save and may quit, updates fail, and the Mac can struggle to log in or start. Files aren't erased, but free a few gigabytes immediately.
Things start failing: apps can't save or may quit, documents can be lost mid-save, Mail and Photos may stop working, updates fail, and in bad cases the Mac struggles to log in or start up.
Your files aren't erased by a full disk, but the risk of a damaged save goes up. Free a few gigabytes as soon as you can — delete large, obviously unneeded files, empty the Trash — then make proper room. See if the Mac won't start.