Locums completing the 'Registrations' tab of their profile sometimes get blocked from continuing or saving the Profile - this is due to the GMC Number already being in use.
Every User Account is linked to a unique email - you use it to log into your account. If your Email and Password combination don't work, it might be the email! (but do check for typos either way)
These days, its easy to forget which password is used for every system out there - you can reset it whenever you wish