Your problem is because you are not using quotes around your UserId.
So, change the following line:
string sql = "SELECT * From Personal WHERE UserId = " + comboBox1.Text;
To:
string sql = "SELECT * From Personal WHERE UserId = '" + comboBox1.Text + "'";
Assuming you have complete control over the values that are placed into the combobox.
Personally, I would use SqlParameters for this, as it would help prevent SQL injection issues.