SELECT COUNT(*) FROM MyTable
will return the number of rows: if it's zero (and you can use SqLConnection.ExecuteScalar to get just that result) there is no data in the table.
But ... I'm not sure you understand this stuff: a database table will normally have more than one row, and a textbox is pretty much a "single item" display element; it isn't good at showing multiple rows.
So I think you want to go back a step and work out exactly what you are trying to achieve here: normally a DataGridView or similar is a better display choice for a table full of data!