The cron semaphore is probably locked. You could try calling drupal_cron_cleanup() from anywhere in your code (that doesn't happen to be invoked by cron) and that should unlock your cron semaphore variable.

If you have drush configured you could also try:

$drush vdel -y cron_semaphore

The cron semaphore is probably locked. You could try calling drupal_cron_cleanup() from anywhere in your code (that doesn't happen to be invoked by cron) and that should unlock your cron semaphore variable.

If you have drush configured in Drupal 6 you could also try:

$ drush vdel -y cron_semaphore