log on failure
 

brand new 2 Ubantu & set up standard Ubantu compartment accessed via 1 user name only and password.
1st few times all good but now suddenly, unexpectedly password declared invalid.
Had written down password so it is correct & not entry error.
Not know how to reset password or bypass 'username/password log on screen'
Am on an Acer 5542G with windows 7 home premium.

Root password not working would be a matter of concern.
When you get to grub do you see an option of safe mode? If you see it then select that. The next screens would give you a couple of options including changing password. At the prompt you can change the root password.
The other option (if you do not see safe mode) is add the entry 'init=/bin/bash' to the line that begins with the word kernel.
Let us know if this helps.

Here is an easy to follow tutorial for resetting the password from recovery mode in Ubuntu: http://www.psychocats.net/ubuntu/resetpassword
Whn setting the password, remember that linux is case sensitive.
Write back if you need more help.

log on failure
 

Thanks!
sorted
very impressed with the prompt and helpful advice
which worked perfectly
:D