Did you receive your new SIM card through the post from us? You can register the new SIM card on our online form to get you up and running with the same number.
Did you report the phone lost to our customer service team? They would have put a block on the IMEI number on the device to blacklist it and prevent anyone using it if it was found.
To unblock the phone itself, you would need to speak to our customer service team. You can complete this over the phone once your SIM card is active, or you can do this with our Live Chat team from your My EE account.