Like the service is there but it's just filterd enabled for lan access only .. what will happen ?
Yes, it's possible to DDoS even if no ports are open.
Keep in mind that a DDoS attack works by exhausting a resource. It can be processing power, storage, memory, or bandwidth. So if an attacker sends way more traffic in your way than your network can receive, your network access will be severely degraded. No matter if there's a service active to receive the packets.
The most effective way to counter such attack is to work with your upstream ISP and tell them to drop every packet assumed to be part of an attack. That is not cheap or easy, but is how most internet services protect themselves.
The other way is to increase the network bandwidth to have more resources than the attacker, but this is more expensive than the former solution.