I had a rootcanal 8 years ago an now its absessed the size of my thumbnail in the front of my mouth.. why is this if I had the rootcanal how can a absees happen

This is something you'll need to discuss with your dentist.

This is something you'll need to discuss with your dentist.

I did cause my crown fell out an they wanted thousand more dollars to do it again so I sought out another dentist an that's when he saw the quarter size abscess.. if all the nerves are extracted how can it abscess?

It can get infected for other reasons that have nothing to do with the presence or absence of nerves.

Did he prescribe antibiotics?

Abscesses happen because there is an infection in the tooth. Doesn't matter if there is nerve tissue or not. If the abscess never healed from the original root canal (the only way to tell if via x-ray), then most likely, the dentist missed a canal or left some tissue/bacteria in there. Teeth are tricky, textbooks say that tooth "X" is supposed to have a certain number of canals located in certain areas, but teeth don't read the textbooks. A tooth that is "supposed" to have 3 canals may actually have 4,5 or 6, and the canals may even have little "communications" between one another. Think of it like the letter H, the two long lines are the main canals and the cross is the "communication" aka accessory canal. There are many places inside a root where bacteria can be safe from all the chemicals/files us dentists use, and if some bacteria remain, they are going to continue to multiply and cause problems. However, If the abscess healed and then came back years later, it's because it got re-infected somehow. The most common reason for the re-infection to occur is because the filling came out, or the crown is leaking/came off. Hope that made sense