Computer Help and Support

Your fan running constantly is a great signal that some process is spinning on your machine, taking all of it's cycles ... and leaving almost none for your Word process.

There are innumerable things that can cause this ... from something moderately innocuous like an "automatic update" that Windows is doing in the background, to some normal program deciding to get crazy and spin hard, to malware running on your machine and trying to mine bitcoins at your expense.

If a reboot doesn't resolve the issue, you should (at least) run "Task Manager" or "Perfmon" to try and identify where your CPU cycles have gone. Running some malware software check would be a good idea as well.

Good luck!