Please note: this example uses Microsoft Word. The process is similar for other Office apps (Excel etc).
Microsoft Word automatically saves your documents every 10 minutes (approximately). You can check how frequently the save is running by opening Word and selecting the File tab > Options > Save > and look at the value in the Save AutoRecover information every (number) minutes line.
This does not replace saving your own documents. It is only intended to prevent loss of work if Word closes unexpectedly (e.g. a power cut). Always save your work regularly.
Finding autosaved documents
1. After a power cut or another unexpected closure of Word, open Word again and go into a new document.
2. Select the File tab, then Info, then Manage Document, and select Recover Unsaved Documents from the dropdown
3. A Windows Explorer window will open. In the top bar enter: %APPDATA%\Microsoft\Word
4. You will see a list of .asd (auto saved document) files that you can choose from. If you had already saved your file you should see it with the name you gave it prefixed by 'AutoRecovery save of'
5. Open the autosaved file, then File and Save As, and save the temporary auto saved version it in the proper location and with the file name you want to give it
Other Office apps
The same procedure will work but at point 3 type %APPDATA%\Microsoft\ and search for the folder with the name of the app. Once you go into it the asd (auto saved document) files for that app will show.