Hi,
What is your Portal data source? Is it Web AS Java or is R/3?
If it is R/3 system, please check if R/3 system is up or not.
If it is Web AS Java, please check if J2EE_GUEST user is not locked.
If it is locked, unlock it and reset the password and restart the server if required.
Please also check the space of oraarch folder that might have got full.
BR,
Anurag