DIAGNOSIS Unresponsive buttons are typically caused by moisture or coffee spills seeping behind the control panel and gunking up the membrane switches or connections.
FIX Unplug the machine. Use a slightly damp cloth (not wet) with a bit of mild soap to thoroughly clean the entire button panel, wiping away any sticky residue. Let it dry completely for a few hours before plugging it back in. Often, dried syrup or oils are creating a barrier.
IF THAT DIDN'T WORK The problem is internal. Condensation from steam or a past spill has likely corroded the thin contacts behind the buttons. There's no user-serviceable fix for this. You'd need a replacement control panel, which isn't a practical DIY job for most people.
HEADS UP Avoid wiping the control panel with a sopping wet cloth or spraying cleaner directly on it. Wring out your cloth thoroughly. Water and electronics are a bad mix, even on the outside.