Congratulations on your “new” harp! I agree with Biagio that the string was likely just at the end of its life. Sometimes changes in temperature and humidity can have an effect on string breakage, but just as often it’s random chance!
Did you find the resources you needed for how to change the string? We actually don’t have a video quite yet, but we are working on it as we speak, so hopefully we’ll have that up within a week or so. For now, going by our paper instructions (which are downloadable on the website) is probably your best bet.
Nylon strings are pretty stretchy, and you can often get away with tuning them sharp to help them stretch out more quickly. But as you found out, it doesn’t always work, so the safest thing is to tune just up to pitch, and to keep tuning up to pitch as often as you can until the string has stretched.
Unless it’s damp enough to grow mildew on your harp, you probably don’t need to worry about humidity too much. That’s more of an issue for solid-wood harps, and since your Ravenna is made of laminated wood, it won’t react much to changes in humidity.
I would be curious to hear @Biagio’s reasoning for never leaving a harp in the case. In our 35 years of building harps, we’ve always stored them in cases here in the workshop, and that hasn’t caused any issues that we know of. Many people prefer to keep their harp in a case at home, and there are a variety of reasons, including the ability to use an in-case humidifier to keep a solid wood instrument from cracking in dry weather. Some people prefer to leave the harp out, because they find that they play it more when it’s not all packed away. You can probably use your own judgement about what will work best for you!