do-release-upgrade stopped, and couldn't re-run

I tried ubuntu upgrade from 10.04 to 10.10, and 11.04. It was successful from 10.04 to 10.10, but while upgrading from 10.10 to 11.04, I felt something wrong, and stopped it. And I tried to re-run do-release-upgrade, but failed. Here are error messages.

Checking for a new ubuntu release
/usr/lib/python2.6/dist-packages/apt/progress/old.py:134: DeprecationWarning: apt.progress.FetchProgress is deprecated.

  FetchProgress.__init__(self)
/usr/lib/python2.6/dist-packages/UpdateManager/Core/DistUpgradeFetcherCore.py:202: DeprecationWarning: apt_pkg.GetAcquire() is deprecated. Please see apt_pkg.Acquire() for the replacement.
  fetcher = apt_pkg.GetAcquire(self._progress)
/usr/lib/python2.6/dist-packages/UpdateManager/Core/DistUpgradeFetcherCore.py:205: DeprecationWarning: apt_pkg.GetPkgAcqFile() is deprecated. Please see apt_pkg.AcquireFile() for the replacement
  af = apt_pkg.GetPkgAcqFile(fetcher,uri, descr=_("Upgrade tool signature"))
/usr/lib/python2.6/dist-packages/UpdateManager/Core/DistUpgradeFetcherCore.py:208: DeprecationWarning: apt_pkg.GetPkgAcqFile() is deprecated. Please see apt_pkg.AcquireFile() for the replacement
  af = apt_pkg.GetPkgAcqFile(fetcher,self.uri, descr=_("Upgrade tool"))
/usr/lib/python2.6/dist-packages/UpdateManager/Core/DistUpgradeFetcherCore.py:209: DeprecationWarning: Attribute 'Run' of the 'apt_pkg.Acquire' object is deprecated, use 'run' instead.
  if fetcher.Run() != fetcher.ResultContinue:
Failed Upgrade tool signature

Failed Upgrade tool
Done downloading            
Failed to fetch
Fetching the upgrade failed. There may be a network problem. 

Is there any way to re-run do-release-upgrade, or reset upgrade status?