You could try this, go to the login page here:,
just under where you input your password you can click on "forgotten password". This takes you to a page where it says "find my account" where you can enter "Email, Phone, Username or Full Name". Try entering one of these (My email gave the best result for me) and follow through the instructions to reset your password so that you can access your account.
If this does not work there are a number of options here depending upon what you have lost, ie you can't log in, your facebook page has been locked and you can't change it, or it no longer appears to exist. I could go through them all but it has already been done here:
How To Recover Your Facebook Account When You Can No Longer Log In
It details the methods you can try depending upon the exact circumstances.
Hope this is helpful, if so would you please register that with Fixya