[SOLVED,,but still is mysterious] HELP: session problems

[eluser]akkumaru[/eluser]
HELP:
I just came up this morning to find that I cannot login to my own application,,
most (if can't be said 'all') of the session codes in my applicaton did not working, even the simplest code like session->set_flashdata() won't work,
and it was just OK last night,,

so what can be the problem,,??