help with restoring a password

[Trevor Nye]

I have shown a few people Ubuntu running on my Toshiba laptop, which has a
French keyboard. The version I was running is in English. Without my
observation (dimwit) a young laddie changed the password for me. Now I can
not get back in. I can get the root/password ok. But I have no idea how to
recover from the command line.

Any help would be greatly appreciated.