Thank you Farrukh bhai for your reply. But I have already found a solution that worked for me.
Since the problem had started after a power failure, therefore it was clear that the most probable reason for this error was some corrupted files.
So I opened Synaptic, selected "Repositories" from "Settings" menu and disabled all the repositories. Then I clicked the "Restore Defaults" button in the "Authentication" tab. Then I reloaded the package manager in order to make sure that the cache was completely deleted, re-enabled the repositories and reloaded again.
And the problem was gone.
The only problem now is that the download server has been set to main server and I cannot find out the server for Pakistan in the list. But since I am a dial-up user, therefore it is not going to effect me too much.