There are many factors that influence the answer to this question. It can come down to the age of your site, how your business has changed, or it is just not performing as it used to.
Your site is old and tired
Like anything, fashions and styles of web design change from year to year, but when you add the fast pace at which web technology changes you can find your website can feel old and out of date quite quickly.
Old and tired doesn't always mean the look of the site, often it can be your content that is stale and needs a refresh. Keeping your content up to date is important not only to keep your customers interested, but it also helps with your search rankings.
While you're updating the content take some time to refresh the images too. Keeping the visual elements of your site refreshed can really life your websites appeal.
Your business has changed
Most businesses are not static organisms, they grow and change, and often the website is forgotten.
If you have added (or removed) products or services from your kit bag it is important that this is reflected on your website. You could be missing out on new opportunities, or as I came across recently with one business, you could still be getting enquiries about services you no longer offer - which is just wasting your time.
You may have updated your branding, changed your logo and colours, so this is the perfect time to give the website a refresh too, and not just swap the logo and some colours. Turn your brand refresh into an opportunity to share your new look with your customer base and remind them that you're still there.
Your competitors' site is better
This is one area where keeping up with the Jones' is a good thing - or taking over the Jones' which is even better. There are very few (and lucky) businesses that don't have competitors, and it is good business practice to keep up with what they are doing.
If you see your competitors doing better than you it might be worth spending some time to find out why. It could be that they're updating their content on a regular basis, they might be blogging to help boost their search rankings, or it could be that their website provides their customers with a better user experience.
As the end of the year is fast approaching it could be good to spend a bit of time over the holidays to review your website and see what your competitors are doing, and put in place a plan to either update and add to your content or look at a complete website design overhaul - especially if your site is 5+ years old.