If your Drupal cron fails to run then most probably the process is stuck and needs to be fixed manually.

You will have to open the common.inc file in a text editor (for example through cPanel->File Manager). It is located in the includes sub directory of your Drupal root folder. Change the following line from:

$semaphore = variable_get('cron_semaphore', FALSE);

to

$semaphore = FALSE;

After that save the modified file and go to the Reports->Status report page in your Drupal admin area. Run the cron manually. After the successful execution you may revert the changes you made in the common.inc file.