Empty / missing value in input fields – textbox, etc (best practice)

I'm working on a client application with a lot of different forms displaying a data retrieve from the server or from other source.

In some occasions some parts of the data is missing or was failed to retrieve (some corrupted data or just optional data).

Now I find my self with forms (view with inputs like textbox) with empty fields, which of course doesn't look to good.

What should I display in these empty fields? Maybe I should use some constant string like "Unknown" or "-" or "?" , or maybe I should hide the empty textbox complete from the view?

Is there any guidelines or standards for this problem?