Removing a transparent background

[Pallabi5EEF]

somehow, a few section of my drawing become transparent. I need a solid background for the whole picture . How can I do that ???

[lucykafka]

Look at the lowest level, did you make it invisible by accident?

[Sjaani]

The first layer of a new file should be white as a default. If you have accidentally turned this off as mentioned by lucykafka or you have accidentally deleted it you can create your own new bottom layer.
If you need a single solid colour background the easiest way would be to create a layer. Make sure it is at the bottom of the stack then fill it, either with vector or pixel colour.