The battery isn't seated correctly or it's damaged. Insert the battery cartridge all the way until it locks. If you can see the red indicator on the button, it's not locked. Push it fully until the red disappears. If the drill still won't turn on, check the battery terminals for debris. Do not touch the terminals with any conductive material. Try a different Makita BL18 series battery cartridge from the approved list: BL1815N, BL1820B, BL1830, BL1830B, BL1840B, BL1850B, or BL1860B. Using any other battery can cause injury or fire.
IF THAT DIDN'T WORK: The tool switch might be faulty. Do not use the drill if the switch does not turn it on and off. It needs repair by a qualified technician.
HEADS UP: Never use a damaged battery. Store batteries away from metal objects like nails or coins to prevent shorting the terminals.