Can't log in?
If you can no longer login to a PC that you used to be able to then the most likely cause is that you have exceeded your disk quota. There are 2 ways to check this:
1. get a friend to login to your computer. If they succeed the problem is your quota or login status
2. Try to login to another computer (ideally one that you have seen in use). If you cannot login then the problem is you.
Assuming the issue is quota then although the graphical login fails you can still either ssh in from a personal device or attempt a command line login from the computer that you are sat at. If you are unable to do either of these things then please contact the IT team.
Password is rejected
If you are sitting at a Maths computer, make sure there isn't a spurious space before your username as this will prevent you logging in. Remember that for most people your Maths account is your crsid (never your email address). Your maths password is not your University Account password. It can be set or reset in the self service.
If you have left the CMS and your password has suddenly stopped working, it is likely your login has closed. If you need a login after you leave then refer to our leavers page:
https://www.maths.cam.ac.uk/computing/collaborator-access
If you are a member of the department you can check the status of your maths computer login in our selfservice.
Not authorised to login
Either you are trying to log into a machine which is neither public-access nor in your group, or if you are no longer at the CMS, your access may have been restricted to ssh.maths only. Email help@maths who can easily supply you with access to the machine on your desk.