The PC in question is not a laptop, but an old Dell Dimension Mini-Tower, so the only 'battery' is likely to be a CR2032 coin cell for CMOS backup.

If the coin cell has discharged, which is quite probable in an old machine after a few weeks without use, the CMOS could have defaulted to its factory settings (not necessarily the settings the PC was shipped with!), but you'd get a message saying so with the option to go into the BIOS setup to change them.

In practice, the BIOS would normally default to auto-detect the hard drive (and CD Rom), so the machine should still boot if the drive is healthy and bootable. The fact that it doesn't means the drive (or less likely the main board) is faulty.

The usual cause of a hard drive failing after a period without use is 'stiction' (short for 'static friction') - the motor is prevented from spinning up because the heads have literally stuck to the platter(s). Heating, freezing (or even patience!) can sometimes rescue such a drive, though it should be backed up and even replaced as soon as possible if it can be coaxed back to life.

BIOS = Basic Input Output System - the information the mainboard needs to be able to find the hard drive, amongst other things

CMOS = Complementary Metal–Oxide–Semiconductor, which refers to the chips which store the BIOS setup information.

;o)