Going from plain text back to Endnote bibliography

Is it possible to convert a document with plain text citation and bibiliography to be linked again to Endnote? How?

Not easily.  Do you have a backup, even a slightly older one?  you can convert that to unformatted and then compare it to the current version, keeping the citations (rejecting the plain text version) and accepting the other changes.  

Are they numbered or author, et al year?  If numbered, you could create a new library, where you drag them in in order to it (so the record numbers match the citation numbers in the document) and then convert the numbers to # in temporary citation delimiters.  If author year, you can clean them up to remove the et all part and put them in delimiters, and let endnote find the ones that match, and select the correct ones.