Expanding a pattern fill but then the object goes blank and the pattern disappears.

Your pattern is not visible in the Swatches panel and it fills at an angle.

I had to do what Anna did to add the pattern to the Swatches panel.

I selected your object and clicked the + sign in the Swatches panel to add the pattern swatch.

I deleted your object and dragged the pattern swatch from the Swatches panel to the artboard.

It is very large, so I scaled it to 10% .

Another thing I noticed is the structure of the pattern.

Normally it shows as a group with an empty rectangle at the bottom of the group.

This one is a group with a clipping group that has a clipping path at the top and below the clipping group an empty rectangle.

The clipping path seems to do something strange so I deleted it and dragged the group  back to the swatches panel.

After that the pattern fills and expands normally.

I could not find the source of this pattern 🙂
But there is a workaround.
select the object and in the Swatches panel choose "New swatch" (with the button or the menu item). Then the pattern appears in the Swatch panel, and you can edit it by double-clicking on the swatch or expanding the object to edit the particular unit.

Did you try to double click the pattern swatch in the Swatches panel to check it in the Pattern maker?

After Expanding, do you see something in Outline View?

Is it your own pattern or it's from any stock? Sometimes the stuff from stocks has some internal issues. And, as Ton said, it would be very helpful to see the working file.

Is it possible to share an example file? Dropbox, CC filesharing, WeTransfer, Google...