At 95% capacity Windows thrashes. Index updates queue, SSD wear leveling struggles, everything feels broken including search.
I hit this on a work laptop last spring. Filename search stayed usable with a few habits until I cleared proper space.
Short-term fixes
Exclude largest unsearchable paths — video renders, ISO dumps — from index so watcher stops touching them. Clear temp, empty recycle bin, run disk cleanup.
Don't disable search
Some tools let you move the index file to another drive with free space. When disk is full you need find-by-name more, not less. Fix space, keep the tool running.
See also: When not to index a folder, SSD, NTFS, and why search feels instant now.