Anything more than an ounce per pound of milk chocolate is toxic---so if your dog ate more than 40 oz it's toxic and you need to go to the ER (you didn't mention how thick the bar was and for reference a regular Hershey bar is 9 oz).
If it was semi-sweet/dark chocolate or baker's/unsweetened chocolate then it's even worse. If it's within 2 hours of ingestion then your dog can be made to vomit either at home or at the ER and the effects of the toxicity will be lessened.
Now if the level of theobromine in the chocolate was not enough to cause toxicity you are not out of the woods. Many dogs develop gastroenteritis (upset tummy/diarrhea) or even worse pancreatitis (inflamed pancreas) that could require hospitalization depending on the amount ingested and if your dog has an iron gut. Labs tend to be more of the iron gut variety.
If there are any questions I haven't answered here, please feel free to contact me. You can also look up "chocolate toxicity" on petplace.com, a very reputable veterinary website. Best wishes.