I lost one canary overnight and now my second one is very ill. Idk what is causing it...

I started taking my Canaries off their seed heavy diet 2 days ago to transition them onto pellets, and I was feeding them greens from my garden. All of a sudden, my male canary died overnight, and there were no signs of illness. I woke to him this morning deceased on the bottom of the cage. Now, my young female is visibly unwell. I immediately cleaned and prepared the quarantine cage. I have put fresh water and seed. I have her sitting in front of the heater covered with a blanket over the cage. It's a public holiday here today, and the vets are closed. I worry that she will be gone by then. I hope that the simple act of keeping her warm, fed, and in the dark will help her pull through.

I honestly dont know what has caused this sudden illness in my birds. They are kept in a very large cage that is cleaned weekly, provided fresh water daily, and a variety of greens and veggies. And lots of enrichment activities like shredding toys, mirrors, a bath etc.

My only theory is that maybe some of the greens I had from my garden were poisoned without my knowledge by another family member. I'm highly against pesticides for this exact reason. Is there anything I can do or give her to help her pull through until the vet opens again? I am in a very rural area, so I have very, very limited options, unfortunately..

I am very clearly upset by this situation, so your empathy and understanding would be very much appreciated.