Recovery depends mostly on the file type, not the fact that the camera was in Manual mode. If you only have JPEGs and the photos are badly blown out, there usually isn’t much detail left to recover. JPEG files discard a lot of image data, so once highlights are clipped, editing software can only do so much.

If you have the original RAW files (such as .NEF or .CR2), you have a much better chance. RAW retains far more sensor data, so you may be able to pull back some highlight detail in a RAW editor.

So in short:

• RAW files: possibly recoverable, at least partially
• JPEG only: likely very limited recovery if the images are almost completely washed out

You can still try lowering exposure/highlights in your editor, but if the bright areas contain no detail at all, they can’t really be restored.