DIAGNOSIS Sticky buttons are a classic sign of grease and steam infiltration behind the membrane of the control panel, especially on a unit that gets heavy use.
FIX Unplug and let the unit cool. Use a cotton swab dipped lightly in isopropyl alcohol (90% or higher) to clean around the edges of the Start button. Press the button a few times while the area is damp to wick some alcohol into the seam. Wipe away any excess and let it dry completely for 15 minutes before plugging back in. The alcohol displaces grease without damaging the electronics.
IF THAT DIDN'T WORK If the button feels physically stuck down or other buttons are also failing, the control panel membrane is likely damaged. The whole top control assembly would need replacement via Ninja support.
HEADS UP Avoid spraying cleaners directly on the control panel. Wipe it down with a barely damp cloth when cool to prevent this.