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my problem is as following: unable to add/remove any program using either update-manager or Synaptic Package Manager or terminal

1. update-manager is asking for partial upgrade and while updating software-center 5.6.2 catalog , there is no progress beyond the line "this is may take a moment"
2. synaptic is unable to obtain an exclusive lock, similarly can't do terminal command
   sudo apt-get update

3. if i try to break down the lock using the command

   sudo fuser -cuk /var/lib/dpkg/lock; sudo rm -f /var/lib/dpkg/lock

   it turns off my monitor display and i have to restart the whole system.

   note: this whole trouble started ,when i found ubuntu software-center missing after adding a repository and reinstalled it.

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