Since Nero CD-DVD Speed is unable to read the suspect DVD in two different devices it is safe to assume the DVD disk is unreadable.
In seeing that the suspect DVD was Burned a year ago the problem is most likely caused by something referred to as DVD Rot. In essence the DVD media has deteriorated to the point of unreadable.
I am not going to attempt to explain what DVD Rot is but suggest doing a Google (http://www.google.com) search for the term DVD Rot.
This problem of DVD Rot is a prime example why if the data you are burning on to a DVD is important information to you it is important to use top of the line quality reliable DVD media.