DIAGNOSIS This is almost always a water level issue. You're either using too much water or not enough, which prevents the pressure from building correctly to push the coffee up.
FIX First, let it cool down completely. The trick is the safety valve inside the lower chamber. 1) Fill the lower chamber with cold water only up to the bottom of that little brass safety valve, never above it. 2) Fill the filter basket with ground coffee, level it off, but don't pack it down. 3) Screw the top on tightly. 4) Make sure the power switch is in the 'O' (off) position before plugging it in, then flip it to 'I'. The coffee should start flowing into the top chamber in 4-6 minutes.
IF THAT DIDN'T WORK Check that you're using the right grind. If your coffee is ground too fine (like for espresso machines), it can clog the tiny holes in the filter basket and stop the flow. Try a medium-coarse grind, more like for a regular drip machine.
HEADS UP Never, ever open the pot while it's hot and under pressure. That hissing steam can cause serious burns. Wait until it's completely silent and cool to the touch before unscrewing it to clean.