Lava lamps were invented in the 1960s, but their mysterious flowing behaviour has maintained at least cult popularity long afterward. Lava lamps are fragile items. Shaking, improper heating, incorrect concentrations in the liquid solution, and simple ageing can prevent proper function. Sometimes, the wax "lava" just sits dead at the bottom of the glass globe, unwilling to rise into its usual globular form. More than one problem can cause a "dead" lava lamp, so you may have to explore a number of potential glitches before you solve the problem.
Check the bulb, if the wax lies flat at the bottom of the glass globe. The wax is likely not heating up enough. The bulb may be broken or may emit inadequate wattage.
Replace the bulb using the guide given in the Generation Store link in the References section. If the bulb is merely broken, use the size stamped on the side of the bulb to find an exact replacement. Otherwise, use the table to find the next hottest bulb size.
Turn off the lamp if the wax has a dome shape to it, as opposed to lying flat. The dome shape indicates that the wax is overheated and was left running too long. Don't operate the lamp for more than 10 hours at a time. If the problem repeats, consider replacing the bulb with a lower wattage.
Operate the lamp in a room between 68 and 73 degrees. Don't place it on a TV or near the air conditioner. Doing so can stop its normal flow. If none of these corrections so far can solve the problem, continue to the next step.
Heat the globe up fully by running the lamp for at least four hours. Remove the globe from its stand and place it on a flat surface. Use potholders to do so, since it will be very hot.
Turn the globe very slowly by its cap (which won't be as hot as the glass). Do this for about a minute. The aim is to get the wax to move relative to the coil in the bottom of the globe.
Return the globe to the base and turn on the lamp. The flow should pick back up again after an hour of heating.