It is quite common for a chimney crack to occur in your chimney. Most of the time a crack in your chimney is caused by water, but a lightning strike and improper construction can also be the cause.
Water entering the crack will slowly cause the crack to worsen due to freezing and thawing causing it to expand. Preventing moisture from getting between the masonry is important.
You should not ignore a cracked chimney because there can be potential danger if left unattended. There is a possibility that the structure of the chimney can falter causing it to lean and eventually collapse. You may also encounter toxic fumes seeping into your home if the chimney flue is cracked also.