Where do I start?
Firstly, don't do it like that! 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. Use Parametrized queries instead.
Secondly, this test is redundant:
if (ds != null)
Because you always do this before it:
DataSet ds = new DataSet();
can never be null.
Thirdly, don't use magic numbers! If you are going to use absolute values to access a single row, then you have to test first to ensure that there is a row at all:
label128.Text = ds.Tables.Rows["Name"].ToString();
You don't check, so if there are no rows where the phone number is an exact match, you will get the exception you show. Check the row count, and don't proceed if it is zero.
And BTW: Do yourself a favour, and stop using Visual Studio default names for everything - you may remember that "TextBox8" is the mobile number today, but when you have to modify it in three weeks time, will you then? Use descriptive names - "tbMobileNo" for example - and your code becomes easier to read, more self documenting, easier to maintain - and surprisingly quicker to code because Intellisense can get to to "tbMobile" in three keystrokes, where "TextBox8" takes thinking about and 8 keystrokes... "label128"? That's going to be a very, very busy UI you got there...