Trying to restore from a Time Machine back-up

Had bad HD replaced on my iMac by Apple. I updated the OS to Mountain Lion, boot in restore mode, start the restore from the last back up, get the "restoring..." dialog box with the progress bar, but it just keeps running and never finishes. Neither the iMac internal drive nor the external drive I use for Time Machine back-up seem to be doing any read/write activity. I even let it run over night justto see and nothing. Any ideas on what might be going wrong?