Press the spacebar a few times. Press the spacebar and the trackpad at the same time a few times. Randomly touch keys, move your finger on the trackpad.
If that does not work, try plugging in a keyboard or mouse, USB device or iDevice (iPod, iPhone, etc).
Sometimes if the PB has a very low battery it will not wake up from sleep at all until you plug in the power cord. If the battery level is extremely low you may even have to wait to get enough charge back into the battery to allow wake from sleep.