What happens if the glass in my stained glass window breaks?

It happens. A window takes an impact, an old piece finally gives way, a crack that's been there for years spreads enough to become a problem. One broken piece in a leaded glass window is repairable. It doesn't mean the rest of the window is compromised, as long as you deal with it properly.

What to do right away

If the break is fresh and there are loose shards, carefully remove them and keep them. Wrap them in newspaper or put them in a bag. Don't try to press the pieces back or glue them that doesn't work and it makes the repair harder. I use the original pieces as a colour and texture reference when I'm sourcing replacement glass, so having them is genuinely useful.

Tape over the opening on the inside of the window for now. Just to stop drafts and keep moisture out until it gets repaired properly.

Why not to leave it

An open spot in the lead channel lets moisture in. That moisture accelerates the deterioration of the adjacent lead and softens the putty around the neighbouring pieces. What starts as a single broken piece becomes a more involved repair the longer it sits. The sooner I can look at it, the simpler and less expensive it usually is.

What the repair actually is

I source a replacement piece as close as possible to the original in colour, texture, and thickness. For antique glass, an exact match isn't always possible but a sympathetic one usually is. The broken piece comes out of the lead, the new one goes in, and the lead gets re-soldered and re-puttied. For a single piece it's a pretty clean repair.

If there are multiple broken pieces, or the surrounding lead is in rough shape, it's worth looking at whether a fuller restoration makes more sense at the same time. I'll give you an honest read on that when I see it.