If the column of the indicator fluid in a thermometer separates, the void will render the indicated temperature inaccurate. Here are some ways to remove the void in the column.

Cooling
Place the thermometer in a refrigerator or ideally, a freezer. If cold enough, it should send the indicator fluid to the bulb. See the next steps if a refrigerator or freezer is not available or did not work.

Warming
Pour hot water into a container, or use a blow dryer to gradually heat the thermometer. The indicator fluid will rise to the top to join the other part of the fluid. Allow the thermometer to cool down to room temperature. If this method does not work with the first attempt, continue to gradually heat, and cool.

Shaking
Firmly grip the thermometer near the top, so that the bulb containing the indicator fluid is pointing down. Rapidly move the thermometer in a "fanning" motion snapping of the wrist. If the indicated temperature has been reduced from the last time you checked, continue shaking the thermometer downward. It may require repeating many times before the void in the column has disappeared.