INTRODUCTION
I had this experience before where Sharepoint Designer Workflows suddenly decide to stop sending email.
RESOLUTION
This resolution sounds simple but it in fact fixes it. Thanks to my colleague Tim Allingham who actually found the solution.
Go to Workflow Tasks list of the site where the lists that have the Sharepoint Designer Workflow resides, then go to Settings -> Advanced Settings.
Under E-Mail Notification,set “No” on Send e-mail when ownership is assigned. Click OK.
After that, go back again to this setting page and now set to “Yes” and click OK.
This seems reset the Sharepoint Designer email and fixed the problem.
If this still doesn’t work, it may due to the timer job crashing/corrupted. To fix it, stop Windows Sharepoint Timer service then go to C:\Documents and Settings\All Users\Application Data\Microsoft\Sharepoint\Config\<GUID>. Delete the files within the <GUID> folder, then re-enable the Windows Sharepoint Timer service again. This will reset the Sharepoint Config DB cache and will fix the timer job.
Hope this helps,
Tommy
tommysegoro