It depends upon how much damage has been done by the caries. If it has
eaten its way through the bottom of the tooth, gone too far under the
gums/bone, or eaten into the crotch between the roots, then it is
probably non-restorable caries and should be removed. If it is
restorable, but has violated the pulp/nerve of the tooth, then a root
canal procedure can be done to get you out of pain and save the tooth.
More than likely, you will also need a build-up of the missing internal
tooth structure, perhaps a post, and then a crown. You may also need to
have surgical crown extension if the cavity goes to far under the gum.
Lastly, in some people, caries causes pain but does not damage the pulp.
In this case you may simply need a filling. But what you describe sounds
like nerve involvement.

you will need a root canal treatment most probably....and try to visit
ur dentist as soon as possible....you will need a x-ray to be taken
and if the cavity has extended in to the PULP which lodges the blood
vessels and the nerve fibres then a root canal treatment has to be
done along with antibiotics and analgesics.Hopefully your tooth can be
saved.