Have you made any template changes to the footer recently?
The footer should contain something that looks like this:
Code:
<div class="smallfont" align="center">
<!-- Do not remove $cronimage or your scheduled tasks will cease to function -->
$cronimage
<!-- Do not remove $cronimage or your scheduled tasks will cease to function -->
$vboptions[copyrighttext]
</div>
If that "$cronimage" part is not there then none of your tasks will run. If it is there then it might be worth checking:
ACP -> Statistics & Logs -> Scheduled Task Logs
there may or may not be some errors in there