The words for unable to be fixed are uncorrectable, irremediable, irreparable, or incurable.
The word for unable to be "corrected" (as in made to conform) is incorrigible.
something that cannot be avoided = inevitable
The adjective is indivisible.
Invisible
unavoidable
To recover something, or to retrieve it.
Deaf
Alternate. substitute (verb)
It means to replace something, with something else that is equivalent.
The likely word sought is substitute (something used in the place of something else, or an alternate player in sports).
The word substitute can be a verb and a noun. The verb form means to use something in place of something else. The noun form is a replacement for something which does a similar thing.
Something that is lost and then found is recovered.
Synonyms include flawless or ideal.