If your table have disappeared, I think a sql injection attack have ocurred.
This is posible because you create the sql stament by appending strings that corresponds with what the user have typed
"INSERT INTO khaas (VNumber, VColor, VType, VBrand, VEDate, DaysLeft, OName, CivilID, Telephone) VALUES('" + txtKVNumber.Text + "', '" + cboKVColor.Text + "', '" + cboKVType.Text + "', '" + cboKVBrand.Text + "', '" + dateTimePickerKhaas.Value.ToString("MM/dd/yyyy") + "' ,'" + txtKDaysLeft.Text + "','" + txtKOName.Text + "', '" + txtKCivilID.Text + "', '" + txtKTelephone.Text + "' )";
If, for example, the user type in the txtKTelepone somethig like:
999' ; 'DROP TABLE khass
your stament is converted in a INSERT followed by a DROP TABLE.
The way to solve this kind of attack is
never, never, never use the user entrys by appending to string, instead use parameters in your sql stament:
String query = "INSERT INTO khaas (VNumber, VColor, ... blah blah... , Telephone) VALUES(@vnumber,@vcolor, ... blah, blah... , @telephone )";
SqlCommand command= new SqlCommand(query,con);
command.Parameters.AddWithValue("@vnumber",txtKVNumber.Text);
command.Parameters.AddWithValue("@vcolor",cboKVColor.Text)
... blah, blah..
command.Parameters.AddWithValue("@telephone",txtKTelephone.Text);