I have installed testlink it is a wonderfull tool and easy to use

I have uploaded around 200 testcases and when I am trying to add more test cases suddenly the navigation window got blank.
I have tried to debug into the issue and found that in
file "functions/treeMenu.inc.php" the function "function invokeMenu" is returning blank data and hence the "Edit Test cases" link is displaying blank navigation bar on the left without displaying the tree.
Please help me if anyone know what's the issue.
Thanks in advance

Narsing