There are a few things that can cause this problem, but the one I encouter most frequently is that the task sequence has a reference to a software package that cannot be found.
For example, say you are deploying Acrobat Reader as part of your task sequence, and the task sequence references a package called XYZ006F. However, if for some reason the package XYZ006F has been removed you will see the behaviour you describe above. My first step when troubleshooting this issue is to confirm that each package is copied/updated to a valid distribution point, or just to be safe, update each of the packages, ensuring that they are updating to an accessable DP.
There are other causes for this behavior, lack of ethernet and disk controller drivers being one one them - but as I said, the above is the situation I have encounterd most frequently - probably worth a try.