Ok. When they do a root canal, they're taking out diseased nerves in the root of your tooth, so they don't get infected. Then they can put a big filling, or a cap, on top.
The anaesthetic doesn't work on the diseased nerves, from what I know, but there's not much to feel, anyway.
Do you already have a temporary crown on the tooth in question? If so, they'll just take it out and start the root canal. If not, they'll have to drill.
You do need to tell your dentist about the anaesthetic not working yet. It's dangerous for them to do work that you can still feel (unless that's what you want), as you might flinch etc.
Anyway, yeah, once they get the tooth open, they start filing out the canals. They have a series of tiny tiny files, on plastic handles, and they just try small ones and then bigger ones, to scrape out the bits of nerve left in there, so there's nothing to rot and cause problems under the crown.
If it's a multi-day job, you'll have a temporary filling or cap in after that. (mmmm, cloves) And you'll come back for more.
You'll be worn out afterwards, but not slaughtered. Well, your jaw might hurt, depending on where the tooth is. And of course you'll still be frozen. You want to plan on having quite soft food, obviously.
Are you getting a crown in the end? Or just a filling?