DIAGNOSIS Sticky or unresponsive membrane buttons are almost always caused by liquid or grease seeping underneath the silicone pad behind the control panel.
FIX You need to clean around the button without getting moisture inside. 1) Unplug the unit and let it cool. 2) Dip a cotton swab in a tiny amount of isopropyl rubbing alcohol (90% is best). 3) Press the stiff button down and run the damp swab all around the seam where the button meets the plastic housing. The alcohol will wick into the gap, dissolve any sticky residue, and evaporate quickly. 4) Press the button repeatedly, about 20-30 times, to work the alcohol in and free up the mechanism.
IF THAT DIDN'T WORK If the button is physically stuck down, there might be a larger spill underneath. While risky, you can try using a plastic spudger (like for phone repair) to very gently pry up the outer edge of the button cap to see if there’s visible debris. Often, just creating a tiny gap can dislodge a crumb.
HEADS UP Be super careful with liquids around the control panel. Wipe up spills immediately, and never submerge or run water over the top of the unit where the buttons are.