Try using IF statement.
i.e.
void TrimiteInService(object sender, EventArgs e)
{
Program.Connection.CommandText = "if(@DataIesireService <> 'Produs in service') insert into ProdusReparat(DataTrimitereService,DataIesireService, ProdusID, UserName) values (@DataTrimitereService, @DataIesireService, @ProdusID, @UserName)";
Program.Connection.AddParameter("@DataTrimitereService", txtDataTimp.Text);
Program.Connection.AddParameter("@DataIesireService", "Produs în service");
Program.Connection.AddParameter("@ProdusID", codProdus.Text);
Program.Connection.AddParameter("@UserName", UserName);
Program.Connection.ExecuteNonQuery();
AfiseazaProduseReturnate();
}