You didn't specify the brand or model, but for generic advice, try using a strong solution of white vinegar and water and soak overnight or for several days (if very scaled up).
If you were using hard water in the humidifier, try using a commercial humidifier cleaner such as BestAir products (or similar - there are many to choose from): Amazon com BestAir 3US 6 UltraTreat Ultrasonic Vaporizer Water Scale...
(Make certain it's certified for steam and/or ultrasonic use, if your using that type of humidifier).
Follow directions on box. You will have to let soak for a period of time (hours/days) to allow chemicals to do their thing.
If you are using "soft" conditioned water, just refill with soft water and allow to sit for several hours, then discard and refill. The hardened minerals will re-disolve and will be removed when you flush the unit. For hardened minerals clinging to the sides, etc, just use your fingers to break up, possibly a brush or Dolby pad and break them loose.
(Soft water has just as much minerals as hard water, but in hard water there are mostly calcium based and turn rock hard and in soft water they are sodium based (sodium bicarbonate actually) and have a weak structure).
It's important to keep a humidifier clean of deposits, not just to keep the unit running, but to avoid air quality problems (bacteria will cling to scale buildup, which make excellent breeding grounds for them).
If you use "hard" tap water, clean (at a minimum) once a week. Use white vinegar and water - 50/50 mix. If your using "soft" conditioned water, clean once a month. Same with distilled water.
You could also consider using a small amount of bactericide in the water to prevent bacteria from growing. (Okay for filter type humidifiers, but DO NOT use in ultrasonic or vaporizer type humidifiers
). 1/4 cap full should suffice. BestAir 3BT 6 Original BT Humidifier Bacteriostatic Water Treatment 32 oz