Disappearing logon screen and user locked out

I am routinely being locked out from the use account while being in the
account. If i lock the computer or hibernate it and then try and log in it is
automatically locked. This occurs for the admin account, another admin and
also a standard user.
Along with that another wierdness has appeared where not all the users are
displayed in the logon screen. And when in another user and i check for users
it is there, and sometimes gone, however the account exists again after
restart.
I have removed a user to see if that assists however hasn't helped.
Anyone experiencing similar problems or knows of a solution please contact me.
Many Thanks.