Cat not well and missing in apartment

I don’t know if this is a coincidence: Since I came home from major surgery one of my cats has not been acting well. He’s been resting under my bed. He usually snuggles with me. He looks tired and sad like he wants to be alone. He’s been occasionally getting up to eat and drink water. He has a heart condition. My girlfriend tried to play with him this morning when he walked into this kitchen. Shortly after she woke me up to tell me he is missing. We live in an apartment. The door has been locked. Windows shut. Only his brother responds to food. We checked the entire apartment. I love him so much and I’m so worried