Your previous mission will end with you on the Citadel. You will still be able to complete a select few of the missions that you had before the attack, but mostly the missions have changed. The layout of NPCs has also changed, with many of them flooding the hallways and searching for loved ones in desperation. With all of these new NPCs comes plenty of new side missions, so be sure to explore every inch of the Citadel to get a long list of new missions that you should complete before taking on much more of the story proper.
Head to the Presidium Commons to pick up the Athena Nebula: Hesperia-Period Statue, Citadel: Asari Widow, Citadel: Cerberus Retribution, Citadel: Kakliosaur Fossil, Citadel: Target Jamming Technology, Citadel: Volus Ambassador, Citadel: Wounded Batarian, Hades Nexus: Obelisk of Karza, and Citadel: Batarian Codes side quests from the asari on the blue couch in the lobby, the dead asari sprawled out across the floor, the man and the woman in the corner by C-Sec, the salarian next to the planter, the turian C-Sec officer in the café, Primarch Victus near the Spectre offices, the human and the turian near the apartments, the human sitting on the bench near the café, and the officer in front of C-Sec, respectively.
You can also pick up the Citadel: Cerberus Ciphers, the Citadel: Reaper Code Fragments, and the Dekuuna: Elcor Extraction side quests from the turian C-Sec officer speaking into the terminal, the asari speaking into the hologram and looking out over the guard rail, and the elcor ambassador in the lobby, respectively. Head to Huerta Memorial Hospital to pick up two more side quests, Citadel: Cerberus Turian Poison from the salarian doctor in one of the long side rooms to the left, and Silean Nebula: Rings of Alune from the asari by the hospital entrance. From here, go to Purgatory to get a number of other side quests. You can find an elcor in the lobb who will give you the Dekuuna: Code of the Ancients side quest, and an asari near the steps inside who will give you the Nimbus Cluster: Liberty of Asha quest.
Finally, sweep the docks and the Normandy itself for the last group of quests. You can find the Hades Nexus: Prothean Sphere quest by the C-Sec officer speaking with the man in red in Docks: Holding Area, along with Citadel: Medi-Gel Sabotage from the doctor in white, and Citadel: Inspirational Stories from the salarian filmmaker. Head to the Normandy’s dock and listen to the man near the door that you take to get back onto the Normandy to get the Valhallan Threshold: Prothean Data Drives quest, then head onto the Normandy itself and to the med bay. Speak with the doctor to get the Citadel: Medical Supplies side quest.
