Please, don't do that!
Do not 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. This will not only help save your DB, it will also cure your problem...
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("UPDATE myTable SET PersonalData=@PD WHERE UserName=@UN", con))
{
cmd.Parameters.AddWithValue("@UN", username);
cmd.Parameters.AddWithValue("@PD", DBNull.Value);
cmd.ExecuteNonQuery();
}
}