This was previously discussed here:
Shelf Life of Tequila
In my personal opinion, distilled spirits should not expire. However factors such as light, temperature and
especially oxygenation could be detrimental to the flavor during such a long time. Since you opened it during that 21 year span and are just coming back to it now, I would assume that the flavor was compromised at least slightly. If you never opened it, I would assume it would still taste exactly the same as it always did since you keep it in a cupboard away from light and presumably high temperatures. Evaporation of alcohol can lead to bacteria development too, but this is rare if properly sealed.