The reason the dictionary disappears in Word is because the dictionary they have for looking up definitions is online. If you are somewhere where you don't have Wi-Fi, you are out of luck. The Thesaurus and spell check work, but no dictionary.
If you have Word 2013 you are suppose to be able to add a dictionary from the Microsoft website. They also have instructions on how to add one into Word 2010 by pressing buttons that don't exist in that program.
Microsoft Works had a wonderful dictionary. If you are lucky enough to have works on your computer click on "computer", then click on "Hard Disk Drive C". Open the folder "Programs", then open the folder "Microsoft Works". In there scroll down to "WksDict."
Right click on it, scroll to "Send to" then click on "Desktop (create shortcut). Now you have a lovely dictionary that even gives when the word was first used.
I have no idea why Microsoft tossed this wonderful dictionary. If they don't want it they should offer it as freeware, or as a separate program with Word. But Microsoft is out for bucks and the heck with customers. This lack of customer consideration is
why the company is doomed.