To continue a romance with Miranda, you must import a ME2 save file from when the two of you were together. You may confirm your connection when you rejoin, and then meet in private... Miranda will remember you if you help her out of trouble or give her advice. She's not exactly friendly, but she doesn't hate you like Viceroy Dyson does either. If you want to continue the relationship, import a ME2 save file so that you can continue where you left off. If you don't have a ME2 save file, don't worry about it. It's not necessary to keep the relationship going because ME3 will reactivate your character's romance with Miranda even if you don't have a ME2 save file. However, if you want to go further with her, importing a save file is the only way to do it. When you meet again after rejoining, you'll have a chance to resolve any issues between you. This will also be an opportunity for you to persuade her to leave Los Angeles and come live with you on Earth. If she agrees, you'll both receive a copy of the Renegade Armor upgrade at a discount!