addModal.php

<div class="modal fade" id="addNew" tabindex="-1" role="dialog" aria-labelledby="myModalLabek" aria-hidden="true">

    <div class="modal-dialog" role="document">
        <div class="modal-content">
            <div class="modal-header">
                <center>
                    <h4 class="modal-title">Agregar Contacto</h4>
                </center>
                <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                    <span aria-hidden="true">&times;</span>
                </button>
            </div>


            <div class="modal-body">
                <div class="container-fluid">
                    <form method="POST" action="agregar.php" id="MyFormAdd">
                        <div class="row form-group">
                            <div class="col-sm-2">
                                <label class="control-label">Nombre</label>
                            </div>
                            <div class="col-sm-10">
                                <input type="text" class="form-control" name="nombrecontacto">
                            </div>
                        </div>
                        <div class="row form-group">
                            <div class="col-sm-2">
                                <label class="control-label">Celular</label>
                            </div>
                            <div class="col-sm-10">
                                <input type="tel" class="form-control" name="celular">
                            </div>
                        </div>
                        <div class="row form-group">
                            <div class="col-sm-2">
                                <label class="control-label">Email</label>
                            </div>
                            <div class="col-sm-10">
                                <input type="email" class="form-control" name="email">
                            </div>
                        </div>
                        <div class="row form-group">
                            <div class="col-sm-2">
                                <label class="control-label">Dirección</label>
                            </div>
                            <div class="col-sm-10">
                                <input type="text" class="form-control" name="direccion">
                            </div>
                        </div>
                        <div class="modal-footer">
                            <button type="button" class="btn btn-secondary" data-dismiss="modal" onclick="BorrarDatosForm()"><span class="fa fa-close"></span> Cancelar </button>
                            <button type="submit" name="add" class="btn btn-primary"><span class="fa fa-save"></span> Guardar </button>
                        </div>
                    </form>
                </div>
            </div>
        </div>
    </div>

</div>
<script>
    function BorrarDatosForm(){
        document.getElementById("MyFormAdd").reset();
    }
</script>