Undo Silence in the Waveform Editor Tab

If you've saved the file after doing this, then I'm afraid you don't - the Waveform editor is destructive. A much better way of doing anything like this is to do it in Multitrack view, and if you want to silence part of a track, then you can rubber-band the volume. This can be changed at any time during the process, and doesn't disturb the original file content at all.

Whilst Waveform view is perfectly capable of doing editing (and is effectively where the product started from in Syntrillium days), it's always been safest to perform this on copies of original files, otherwise you have nothing to go back to. Multitrack is entirely non-destructive, and in many ways, far more flexible to edit in.