Whose Online Problem - Plz Help

a minute ago, it was working fine then it refreshed it self and an error came up.

PHP Code:

Fatal error: Call to a member function on a non-object in /home/thunder/public_html/gsu/online.php on line 177 

why did that happen?

[ExAvIoUr]

i checked the line 177. and it has this as it

PHP Code:

        $userinfo[where] = "Viewing <a href='attachment.php?s=$session[sessionhash]&postid=$userinfo[postid]'>Attachment</a> in Thread <a href='showthread.php?s=$session[sessionhash]&postid=$userinfo[postid]'>".$thread[title][$post[$userinfo[postid]]]."</a>"; 

i don't get it