You mean you are being asked for your e-mail address and password. You said "all of a sudden" - previously did you not have to sign in at all - are you aware of having made any changes?
Has your security software updated recently - sometimes there is a conflict with hotmail.
If you cannot remember your password then the only route in is to go into the "help" under the log in panel and follow the instructions there -it will mean creating a new password.
I see that there have been a few instances of late with the same issue on Vista machines.