If you started holding the Option key, and only saw Macintosh HD rather than that and Recovery, then it seems that Recovery is missing or damaged on your computer. Some models of MacBook will start in Internet Recovery if you use Command Option R; although it seems that yours is not suitable.
If you are able to start in Single User mode (hold Command & S at the chime), then you may be able to follow the steps below to change the password. Print them out first, & make sure that you know the name of the user that you wish to change. This won't alter the AppleID name & password that were used to install previous software, so you may still need to erase & (re)install yourself; starting with the relevant install DVD(s) which are usually available from Apple for around $20, if you call with the serial number.
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Start in Single User mode & wait for the :/ root# prompt to appear
type mount -uw / then press enter (the slash is important)
again, wait for the :/ root# prompt
type passwd username
then press enter (swapping 'username' for the name you need to change the password for)
You should see
:/ root# passwd username
Changing password for username
New password:
You now type a password, which won't be shown onscreen, then press enter
then you should see
Retype new password:
type the same new password again, then press enter.
wait for the :/ root# prompt, then type reboot & press enter