* Is your file corrupted on disk? See if you can duplicate it, or move it, or rename it. An error suggests it is no longer usable.
If you can duplicate it, then try to open the copy in PADS.
* Did you save an earlier version of your pcb? Does your software do so automatically? That version may be okay.
* Try deleting your PADS preferences file. (Corrupted preferences can cause glitches.) A new preferences file is created next time you run the software, if the software cannot find an existing preferences file.
* Try opening another PADS pcb file. If you get the same malfunction then the software may be corrupt.
Then you may need to re-install the software package.