MacBook restarted unexpectedly after Word froze, losing work

Reopen Microsoft Word and check whether the Document Recovery pane appears. 

If your assignment is listed, open the latest version and immediately save it using File > Save As. 

Also check File > Open Recent. 

If the document is missing, open Finder, press Shift + Command + G, and enter ~/Library/Containers/com.microsoft.Word/Data/Library/Preferences/AutoRecovery. 

Look for relevant recovery files and copy any you find to your Desktop before attempting to open them.

Check your Mac's Trash and, if you use OneDrive, check your online files, Recycle Bin and version history. 

If you previously set up Time Machine, open the folder where the assignment was saved, enter Time Machine, select a backup from before the work disappeared, and click Restore. 

See Apple's guide: Restore files using Time Machine. You can also consult Microsoft's Word recovery instructions.

Before further troubleshooting, save any recovered work, close open applications, disconnect unnecessary peripherals and perform a normal restart using the Apple menu  > Restart. 

Avoid repeatedly forcing the Mac to shut down unless it becomes unresponsive, as this can risk further data loss.

To help prevent the problem from happening again, check for macOS updates under System Settings > General > Software Update, and update Microsoft Word through Microsoft AutoUpdate. 

If your Mac restarts unexpectedly again, note any error message and follow Apple's guidance for unexpected restarts.