I have used kartik datepicker in yii2 advance application, It was working before I using custom css, not getting any error only input field showing. View Code:

    

    
    field($model, 'project_comp_id')->dropDownList(
        ArrayHelper::map(Company::find()->all(), 'comp_id', 'comp_name'),
        [
            'prompt' => 'Select Project',
        ]);
    ?>
    field($model, 'project_name')->textInput(['maxlength' => true]) ?>

    field($model, 'project_desc')->textArea(['rows' => '6']) ?>
     field($model, 'project_start_date')->widget(DatePicker::classname(), [

                    'options' => ['placeholder' => 'Start date & time'],
                    'pluginOptions' => [
                        'autoclose' => true,
                        'format' => 'yyyy-mm-dd', 
                    ]
                ]);
    ?>  
      field($model, 'project_end_date')->widget(DatePicker::classname(), [

                    'options' => ['placeholder' => 'End date & time'],
                    'pluginOptions' => [
                        'autoclose' => true,
                        'format' => 'yyyy-mm-dd', 
                    ]
                ]);
        ?>


    

        isNewRecord ? 'Create' : 'Update', ['class' => $model->isNewRecord ? 'btn btn-success' : 'btn btn-primary']) ?>
    

    

In the following image form structure is showing: