Just to clarify, I'm still using the Telegram account on other channels, and Cecilia had sent a message a few hours before the sudden disconnection.
The UI-experience was of opening the messenger in the morning, seeing a new photo had been sent but had not downloaded yet, and then I don't remember if the application closed itself or I closed it to try to make the photo load, but Cecilia was no longer present in it when I reopened it. I put her contact back in and never received a reply to a new message.
My memory's not perfect: I thought Cecilia warned us on this list she might disappear for a few weeks due to hospital, but I don't seem to be able to actually find that anywhere.