Black dots on back of mirror

Cerium oxide powder you mix with distilled water into a slury. Applying it with a high speed buffer and plenty of time is about the biggest gun there is to attack mirrors and glass with scratces and discoloration.

The silvering of the mirror may have bubbled off.

Joyce is this the front or back of the mirror. If front check out the back that is more than likely where the problem is. You may have to do a a little "demo" to get to it. I know antique or older mirrors which I have do this occasionally . You will then have to google it. like Dfm said the silver is probably gone.

Have you considered giving the entire mirror a make-over with Krylon Looking Glass Spray Paint? (I prefer it to Rustoleum's). It will give the mirror a mercury glass look. Though it won't really be a mirror, it will give you a more uniform look and can be quite striking since you can't remove the original mirror to have it replaced. I have used this on table tops that are too scratched for my taste, on clear glass vases or bottles that catch my eye, and on mirrored sconces for a more romantic effect.

Boy, by the looks of it, it looks like when it was installed they used silicone caulk which can effect the silver. doesn't it look like the pattern that someone would do using a caulking gun?