It's very likely that you have experienced neck stiffness at some point in you life, and if you are unlucky, you might be a frequent sufferer. Neck tightness and stiffness could be caused by a tight or spastic muscle, a misaligned joint, arthritis, nerve impingement or compression, or even by your pillow, just to name a few.
So lets take care of some basics that you have much control on:
Are you waking up in the mornings with more stiffness than the night before?
If so look into your pillow and the support it's giving you, or better said, denying you. In another word, if your pillow does not keep your head and neck level with the rest of your spine throughout the night, you are bound to have neck stiffness and discomfort sooner or later.
Do you read or work on your electronics in bed or on the couch? Or do you watch TV while laying in bed?
If you answered yes, then the wrong positioning of your neck might be causing the problem. The majority of people watch TV or work on their laptops and tablets with neck bent or propped forward for long periods of time. This could cause muscle strain and eventually changing or reversal of neck curvature leading to acute or possibly chronic neck discomfort. It must be noted that even if you are following the perfect ergonomics while working on a computer, reading, or watching TV, when one stays in a fixed position for long periods of time that alone could act as an aggravating factor and cause problem.
Are you trying to lose your tummy by doing sit ups and crunches?
I am not so keen on sit ups and crunches as I hove found the majority of people stressing their neck and their upper and mid back other than exercising their abdomen. So my best suggestion is not do sit ups or at least replace it with those exercises that are not taking a toll on the neck and back. As for crunches learn how to do them correctly (keep your neck neutral and level, don not bend it, and just raise your shoulder blades about 2-3 inches off the ground for every crunch).
If the above modifications do not solve your neck stiffness you are advised to see your chiropractor or primary care provider to look into the other possible causes of your neck pain, stiffness, or aches.