Cron run exceeded the time limit and was aborted.

When run cron manually by visiting mysite.com/cron.php can see blank page

Check the logs, and id get the following message:
Cron run exceeded the time limit and was aborted.
There may be some issue in module

You can debug wehre cron is bailing,
Try This,
In module.inc   , function module_invoke_all

add this extra line:

foreach (module_implements($hook) as $module) {
    $function = $module .'_'. $hook;

    if ($hook == 'cron') watchdog('cron', "hit $module cron");   // add this line

You should be able to check out your watchdog log and see which module is killing cron.