I preserved my DNA in -20C for more than one year and then I ran it on gel to see the vitality, few DNA show very bad bands. Does that mean the DNA got degraded and if so what can I do to keep it vital again?
If you are seeing many bands for DNA that used to show only one band, then the DNA probably either became degraded or contaminated. If it is a plasmid and at least a little of it is intact, you may be able to transform cells with it and recover it using a selectable marker. Or you might be able to use PCR to amplify short stretches of it and then ligate them together. It might be easier to isolate more of it and store it at -80 this time.