This really needs no introduction. So we'll get straight in... Search Engine Optimisation.
Yeah your site was optimised three years ago and you were doing well, but things change quickly. Google (there's only really Google right?) have frequent updates, some tiny, some potentially catastrophic. SEO should go hand in hand with your site - often though its a knee jerk reaction after the event. If your site isn't performing well, its because its not optimised, right? Maybe, but it could be that the content is old (see a pattern here).
Search for yourself on Google. Not your business name, heck if you're not coming up for that things are definitely bad, but for what you do. Be specific, so if its making teapots in Grantham, start there. If you're not ranking, you need to do something about it - is it a rebuild; is it a SEO plaster? That's for you or your Financial Director to advise.