I was having problem completing the quest "Diplomatic Immunity" because I failed to give gear to NPC before entering Thalmor Embassy. As a result, I deleted the quest, thinking I could go to quest giver and get the quest again. Quest giver will not speak to me to start quest again.

Anyone know how I can reacquire the quest (cheat code, etc)?

You "deleted" the quest? How did you do that?

You can't delete quests, not by normal means.

And unlike an MMO, you cannot require a quest by this method anyways. The only way to 'require' a quest is to reload an earlier save from before you have the opportunity to speak to Malborn (sp?)

I wouldn't recommend using the console to restart the quest as that may make things worse, especially depending on how you 'deleted' the quest.

I used the console help to delete the quest(don't recall exact wording), but the quest disappeared from my journal. Now I try to talk to NPC who gave me quest and she does not respond. I don't seem to be able to continue the quest line without first doing "Diplomatic Immunity" quest.

Do you have an earlier save you can load?

Because quite frankly, you've broken the game. Even restarting the quest through the console may make things worse, due to scripts not enabling properly, NPCs being in the wrong place etc.

edit: At the very least, did you at least make a save BEFORE mucking around with the console?

How did you delete the quest?!

By deleting the quest...you literally REMOVED it from the game. Meaning you've literally snapped the game's main quest in half.

Or, as we said back in the days of Morrowind:

I wish they still had a message like this. Most people go back and sandbox after beating the mainquest, so it doesn't make sense to make over half the characters immortal.

To Borinusa:

The console is not a toy, what you do in it is permanent and can cause widespread and far reaching problems to the game. Imagine a house of cards and you pulled one of the bottom cards out. the whole house of cards falls apart. Reload a save from before you removed the quest in the console. If the console command was really able to delete the quest you may have to reinstall the game.

The reason half the characters are immortal is because they're tied to quests, and Radiant AI (Especially back in Oblivion) could lead to situations where the person would unintentionally die. I still remember Fallout 3, and the dying population of Megaton who kept falling off the upper walkways.

Going back to those messages would mean sacrificing the NPC AI so they become mostly static set pieces.