Por que está fazendo um cast para PreparedStatement na linha 4?
O método deve retornar um PreparedStatement, se não estiver retornando, tem algum problema.
Outra coisa, não podemos dizer se estão certas as linhas 6, 7 e 8, pois os métodos getServico(), getValor() e getValorTotal() do objeto os tem que retornar String, Float e Float respectivamente para funcionar.
Está dando algum erro ou tendo algum comportamento inesperado?
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'servico' cannot be null
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
at com.mysql.jdbc.Util.getInstance(Util.java:381)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3536)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3468)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1957)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2107)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2648)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2086)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2371)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2289)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2274)
at acessoAoBanco.FachadaAcessoCadastro.cadastrarOrdemServico(FachadaAcessoCadastro.java:293)
at gui.frameTelaPrincipal.jbFinalizarServicoActionPerformed(frameTelaPrincipal.java:725)
at gui.frameTelaPrincipal.access$400(frameTelaPrincipal.java:35)
at gui.frameTelaPrincipal$6.actionPerformed(frameTelaPrincipal.java:233)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6263)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6028)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4630)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4574)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2475)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)