What is the model number of the motherboard?
Look for a coin cell sized battery on the motherboard. Remove it from the motherboard and measure its voltage. If it is <2.5VDC replace it. Usually it needs to be gently prised out of the battery holder.
The battery type number should be marked on the battery. Usually it is a CR2032 available most everywhere e.g. supermarkets.
Do this with the power disconnected from the computer.
Before reinserting (or replacing) the coin cell battery (usually +ve on top marked on the battery), press and hold the computer's Power On button for a full 30 seconds and then release. This will reset the BIOS if it has been corrupted for some reason, preventing a normal start.
Reinsert the coin cell battery, reconnect the power to the computer and try turning it on.
If it turns on there may be a message saying the date and time is incorrect. This is normal as the BIOS has been reset to factory default settings. Once the date and time has been corrected, the message won't appear the next time the computer is started.
Also any user defined settings in BIOS that have been changed by you to suit whatever operating requirements you needed will have to be set again.