Hey @Aoibh_97068, Welcome to the 3Community 🙂
I'd love to help you here, as you were travelling I would recommend that your device is attempting to connect to the Three Network, you can select your device on our Device Support Guide here and follow the steps to manually select the network.
If your SIM continues to have no service pop through to me here on PM along with your full name, mobile number, address and date of birth and I can check this here for you.