An x ray will give you a perfect diagnosis wether a re root canal is needed or no Sometimes post a root canal the tooth gets infected apically due to some leakage or multiple reasons And a screw is not always needed in a tooth. Sometimes screw is needed only if the tooth doesn't have 3 walls to support And a cap can come out due to cheap cements used Or if the height of the tooth is less Or if you eat something very hard or chewy Or if you break or open a bottle with it

Re RCT cannot be ascertained without diagnosis. You will have to be examined and Xray's taken. The cap will not be threaded by a screw. For any other doubts you may Practo Consult your queries

You need to get the tooth checked. there can be multiple reasons for the pain. Only thorough investigation can rule out if it needs a re-RCT. And only a dentist can know that. Yes, cap/crown can be placed without a screw if enough tooth structure is present.