Are other workflows operating correctly?
If other workflows are not working either (withjout this workflow running) then check that the Microsoft WDynamics Workflow service is running (Start > Run > Services.msc)
If you look in the Windows Application Logs when this error occurs, do you have a corresponding error log entry? If there is, what does it say?
Have you applied any Rollups to the server installation?
There were Workflow issues which were addressed in Rollup 2
This could also be a timeout issue of some description - you could try increasing the SQL timeouts.