I'm so sorry Mike. I know losing a beloved pet is sooo hard.
There is no way for us to tell you what may have happened. As far as using the AC goes...did you make sure it is not blowing on them? Any kind of draft, warm or cold, is not good for them. It can make them sick.
How was she acting before it happened? Was she fluffed up, or unusually quiet? Did her poops look different?
Keep a close eye on your other bird. Do a good thorough cleaning of the cage and all that is in it. Disinfect dishes and toys.
Your other bird will grieve and miss his friend just like you do, so be extra nice to him and spend more time with him so he's not lonely. Do NOT jump in to getting another bird thinking it will help your other one. It won't. Just spend more time with him and take the time to bond with him. If given enough attention he won't need another bird for company. he'll have YOU as his flockmate.
Be sure he is getting a good diet to stay healthy.
Again Mike Mike, I am so sorry for your loss. Take care
R I P Twinkles^