Action disappeared after a restart

@Xuanx 

One should manually save out each action set "folder" individually to an .atn file using the menu on the upper right of the action panel. Unsaved action sets are temporarily stored in the Actions Palette presets file (Actions Palette.psp). This file should be considered temporary and may be subject to corruption or loss, which is why making an explicit save to .atn file for each individual action set is strongly advised. 

If you have a system backup from before the files were lost, it might be possible to recover them using the Actions Palette.psp file.

More here:

https://prepression.blogspot.com/2017/01/photoshop-custom-action-file-backup.html

@Xuanx 

If you saved the Set that the action was in, you can Reload it from the Actions panel menu.

Jane