We have quite a few threads about this subject. I have collected some of them and arranged them by the OS which is primarily discussed in the thread. That is because the exact procedure depends on the OS involved. What's more, since you often need to interact with the boot process, the procedure can vary somewhat based on your exact hardware.
Despite all of the that, there is considerable commonality in the procedures so you should take the time to go through all of the threads. You may get an idea from one of the threads.
You may still need help after you through these threads. If so, help us help you. Give us the details about which OS you have and the hardware you run it on.
Solaris
how do I log into this machine - with a twist...
Lost password on a SPARCstation Voyager
How do I restore the etc/shadow file
Root Password Lost !!!
root locked out
sc password
HP-UX
root pwd
lost root password using (SAM) trusted security
I lost my password root
passwd file corrupted
Boot interaction
Re-enabling a locked root account on Trusted HP-UX
Linux
root and all but 2 users locked out
root password
how to retrieve root paswd
Help! passwd file corrupted
AIX
root account has been locked
Boot in Single-Mode
Tru64
How to Enable locked root account
SCO
Fortgot root password SCO 5.0.
Ultrix
I neede help!!!
NCR Unix
single user mode - user accounts passwords
Windows/XP
Beginners Guides: Forgotten Passwords & Recovery Methods