It's an old computer and haven't used for a couple of months. The laptop is mainly for backup purpose. But now i can't access the computer because of the wrong password. How could i get into the computer again without losing any data?
Make a bootable CD and boot from it, you may have to enter the bios of the laptop to change the boot order, hopefully this will find the password for you.
if you have a pre-created password reset disk, then you can easily reset HP password, but if you didn't have, you may need password reset creator programs like iSeePassword, UUkeys, Datasoft and other software.