Usually an epoxy or glass glue will work. Here's a link with more details:
If all else fails fix it with any glue that will work. Don't worry about glue showing. When completely dry, rub it down a bit and go over the crack with nail varnish. It will show, so why not use a coloured pearl type. You can match up the design on the others if it is one of a few. Make it look as though it was meant to be (Part of a design) eg. Flower stem or tree or add other colours or trails for an overall effect. Sometimes if you can't hide something it is better to make it a feature.....etc.
This might help...it discusses epoxy and heat considerations. https://www.doityourself.com/forum/glass-mirrors/87607-glue-broken-glass-shade.html
Here's a link for a windshield repair kit, same principle:
You can often find replacement glass fixture at a reasonable price. You didn't supply a photo, so any suggestions would be blind.
Apply glue to the edge of one glass piece. Follow the manufacturer's instructions for application methods and amounts.
Press the edges of the two pieces to be glued together. Press them firmly until glue seeps out of the cracks.
Hold the pieces under the UV Lamp. Hold the pieces so that most of the rays shine through the glass and onto the glue. Do not hold the pieces so that the glass seam is directly under the light. Hold the piece under the lamp for the amount of time listed by the glue's manufacturer (approximately 5 minutes).
Scrape the excess glue carefully off of the seam with a razor blade.
Hello. If it’s a clear glass fixture a replacement would probably be the best alternative. A repair would always be visible and likely unattractive.
A crack should not be a problem until you bump the shade. Once glass is fractured it will continue to fracture in the direction of the crack if struck hard enough. You can hide the crack by mixing Elmer's glue and food dye. Paint and let dry. You won't see the crack. There is really nothing that can fix the crack since putting glue ON it is just a bandaid.
I agree with Vimarhonor here. Most replacement shades would cost less than the glue and your time investment.