Naturally, it's a cause for worry, especially after 20 years of long service with your firm. We have no information on the methodology normally adopted by your firm for granting annual increments. Except in some firms that have a policy to conduct 'performance appraisals' individually to consider annual increments, an "average and above performer" could be granted an increment in the normal course. So far, you were given the increment, but this year it did not happen, which understandably should give you worries.
Judge or ask yourself whether your performance has been above normal or lacking. Try to recollect if there was any instance or occasion that could have led to withholding your increment. Why didn't you ask or discuss this with your functional head or immediate boss earlier? They may know the exact reason. As others suggested, you should also ask HR about the reason behind this delay (I would call it that).
Nothing is lost; approach them and ascertain why this has happened to you. You will then realize what you should know thereafter. Don't panic, but if there is anything you should improve upon, don't hesitate to apologize and seek their pardon. Assure them that you will work on it to win back their confidence. I hope everything will fall into place. All the best.