From the Manual:
[quote:3vaj4iw4]Enable autosave
If enabled, Construct 2 will automatically save the project to a .capx.autosave file in the same folder on a regular interval (as determined by Autosave frequency).
In the event of a crash, power failure, or even just accidentally closing and not saving, the project can be recovered from the last autosave by renaming the file with the .capx.autosave extension to just .capx and opening it again.