Once you are sure the batteries are working and properly inserted, you should check the bulb. In a flashlight with an incandescent bulb you should be able to see if the filament is broken or if the bulb has a dark spot that looks burned. After you install the new bulb, carefully clean it with isopropyl alcohol and let it dry completely before turning on the flashlight. Any oils from your hands left on the bulb can cause it to explode once the bulb heats up.
If your batteries have leaked, remove the batteries and stuff the compartment with paper towels to absorb the acid. After you remove the towels sprinkle baking soda inside to neutralize any leftover residue. When the battery compartment is completely dry use canned air to blow out the baking soda while holding the flashlight inside a garbage bag. If the batteries have made too big a mess or it is a cheap flashlight it is best to just buy a new one. In order to avoid corrosion you all batteries should be same brand of battery and replaced at the same time. Never leave a flashlight in a place that is too hot or too humid. These simple rules can help prevent corrosion and leakage, and lengthen the life of the battery and the flashlight.
