kitri, if it a computer a few years old it may have a battery on the motherboard, this keeps the settings like the boot priority and other settings saved
when you next turn your computer on keep tapping del key on keyboard and press the key for recomended settings or if you are feeling daring have a look for the boot priority and put the hard drive at the top of the list
if this works then you will need to go to a shop and buy a new battery
jewelers are expensive but usally the easiest place to pick the batteries up (there a little cell battery)