Hi Guys! Is there anybody who can give me some hint on this?

I have my XOOPS running on RedHat. This morning when I tried to open the site in my browser, it was extremely slow. So I try to restart apache server, but no improvements. Then I reboot the system, everything works fine except that no one can log in. It seems that I disable the cookies, but actually no. Even if I try to use different browser, the same thing happens.

This has happend days before. What I have done that time is reinstall the XOOPS. But this time I do not want to do this. Do you guys have any idea about why this happen and how can I solve this and prevent this from happening again? Thank you so much...