Do you know the root password? As root you can set a new password for your user account.
If not:
Hit esc when booting to get the grub menu (to get the right moment could be tricky, should be after the hardware initialization).
When you see the grub menu press "e" to edit the currently highlighted selection.
Go to the line begining with kernel and edit it by presseing "e" again.
Append "init=/bin/bash" (without the quotes) to the end of the line and press enter. After doing this press "b" to boot.
This will give you a bash session as user root. You can now change the password of the root account with passwd, change the password of your user with passwd your_account or both.