another fine mess -- removed a line from repository - now dysfunctional

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I removed what I thought was a duplicate from  my  repository/other  software /  list - now I can't sudo apt-get update -- and I don't even know what it was I removed. in future  will take note  of such things.

Initially this morning I was working and researching  on how to remove a duplicate  uninstalled  package  that showed up in my CLI  ever since attempting  to  install the game Smoking Guns a couple of days ago . I researched and found out how to  remove  this duplicate  successfully and along the way was sidetracked  into attempting to  install an app and ended up with a missing  key.  I experienced  a missing key after my recent  install of  LL 2.8  last week - but researched  forum and web and easily corrected that problem . Since  then , loving LL - totally seamless  integration with  everything I've installed and every sys preference / change I've made ...  until  today  .  I think what I removed may have  had ubuntu.net  in it's address line and I don't see any like that now. 
I would appreciate  some help with  this . 

after more web search  I think this may have to do with keyrings  but not sure yet .

please bear in mind I am not bash , code, scripting  etc.  literate  .    self learned / taught ---  never had benefit  of being  around  other computer  users or interacting  etc.  -  construction background -  no IT

also please let me  know what form , etc. to make any  attachments / visuals  ; ie.  , leafpad ? , copy and  paste from terminal  and so on . I do know about imgur .    just not completely sure about forum protocol
  thanks very much ,  sorry for all the bloat