DIAGNOSIS The tiny holes in the tip of the steam wand are almost definitely clogged with dried milk and mineral scale. It's a maintenance item, not a breakdown.
FIX First, always purge the wand by releasing a burst of steam into an empty cup right after frothing. For the clog, you need to soak the tip. Remove the steam wand tip if possible (it usually unscrews). Soak it in a cup of hot water with a drop of dish soap or some espresso machine cleaner for 30 minutes. Use a pin or the special tiny cleaning tool that came with the machine to gently poke through each hole. Reattach and run steam into a cup of water to clear any remaining debris.
IF THAT DIDN'T WORK If the wand still sputters after cleaning the tip, the internal steam valve or pipe might have a scale blockage. This would require running a full descale cycle (see the three-light-flashing fix above), as the descaler solution also runs through the steam system.
HEADS UP Get in the habit of wiping the steam wand with a damp cloth immediately after each use, before the milk cooks onto it. That single habit will prevent 90% of these clogs. Never let milk sit inside or on the wand.