How can we fetch the data in the table according to the database table name entered in the textbox? (In C# or ASP.NET)

You can ... it's not even difficult - but it's a very, very bad idea. Let me explain why first ... you should never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.

When you concatenate strings, you cause problems because SQL receives commands like:

SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'

The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:

SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'

Which SQL sees as three separate commands:

SELECT * FROM MyTable WHERE StreetAddress = 'x';

A perfectly valid SELECT

DROP TABLE MyTable;

A perfectly valid "delete the table" command

--'

And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.

So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?

Now, your idea does the same thing: I can type in the textbox anything I like:

Students);DROP TABLE Students;--

Will do the same thing: select the rows and return them, then delete the table.

So ... just because you can do it, doesn't mean you should - and you'll need to add some serious protection and validation on the textbox data or you will lose your DB.

but it's simple to do that:

string sqlCommand = $"SELECT * FROM {MyTextBox.Text}";

Will happily let anyone with access to the computer do exactly what they want with your DB.

I'd strongly recommend you don't proceed with this.