C-Drive decreased overnight from 18Gb free to 3 GB free space.

With only about 3 GB left, move a few large personal files to an external drive first. That gives Windows enough breathing room while the cause is being tracked down.

Since the usual cleanup has not made a difference, the next useful step is to see which folders are actually using the space. A disk-usage tool such as WinDirStat can scan C: and show the largest folders. Running it as Administrator may reveal more protected locations, but use it to identify what is taking up space—not to delete Windows files.

The main places to check after a sudden drop are:

System Volume Information — this can be large because of restore points or shadow copies. Search for Create a restore point, select C:, choose Configure, and check the current usage. The Delete option clears all restore points for that drive, so use it only if losing those restore points is acceptable.

AppData — browser caches, game launchers, virtual machines, Docker, and sync clients can grow quickly.

OneDrive — a large folder may have been made available offline. After confirming that files are safely synced, use Free up space on folders that do not need to remain on the PC.

Windows Update — check Update history and restart the PC. A failed or pending update can temporarily leave a large download or installation folder behind.

If the scan shows that hiberfil.sys is large and Hibernate is never used, running powercfg /h off in an Administrator Terminal will remove it. This also disables Fast Startup, so do not use it if either feature is needed. Do not delete the page file, WinSxS, SoftwareDistribution, or other Windows folders by hand.

If the space disappears again after a restart, note which folder grew and run a Windows Security scan. That will give a much clearer answer than repeatedly clearing Temp or Prefetch.