It could be a number of things. Is this a local or network file?
How big is the file? If it’s very large it may take a while to open due to it running a grammar and spell check. It could also depend on what other applications you have running. You may be using a lot of memory on other apps, leaving less for Word. Provide us with some more details for a better response. Other wise we are providing general tips that may be the cause.