First thing, answer "Yes" to that question. At least, you'll save your work up to this point. Close all open documents and restart Word.
Now, how many documents do you have opened when this happen ?
Apart for some very special cases, you don't need more than two or tree documents open at any given time. Most of the time, you only need to have a single document open. Close those that you don't really need to have open.
After you have finished working on a document, take the habit to close that document.
Since how long is Word open ? There can be some memory leak. Over time, it can eat away all the available memory, even if you have over 16 Gb of RAM available.
Save your work, close open documents and close-restart Word from time to time.
You don't tell if you have a 32 or 64 bits system. You are more likely to encounter that problem on a 32 bits system.
If restarting Word don't resolve your problem, then you need to reboot the computer. This will clear away any memory leak issue as well as any other memory management related ones.
Finally, How much place do you have left on your disk(s)? It's becoming very rare these days, but it's not impossible that you have a saturated disk.
Empty your recycle bin. Delete some old or obsolete files. Uninstall unused/unneeded applications.
If you have more than one drive/partition available, try saving to another drive.
Electro, August 2018
