iPhone 12 battery suddenly went from 35% to 0%

Well, if the battery was fine and suddenly is not, there is a problem with the battery. What that exact problem might be, we can only guess. But, batteries do fail. The message that the battery cannot be verified supports the conclusion that the battery definitely has a serious problem.

Make an appointment at the Genius Bar / Apple Store or take the phone to an authorized Apple service shop, so the battery can be checked and tested. A replacement battery is going to be close to $100 (U.S.)

You can find a list of authorized service shops in your area using this link: Find Locations

The iPhone 12 was discontinued in September 2023, so if the phone was really "new" when you purchased it 8 months ago, the phone sat in a warehouse somewhere for a minimum of 5 months, likely longer. This would not have been good for the battery.

But, if you really did purchase the phone "new" from an authorized Apple dealer 8 months ago, the phone is still under the AppleCare warranty for another 3-4 months, so any service that might be required would be covered by the warranty.

The real bottom line here......the battery is not going work correctly until you take the phone in for service.....but the decision is yours.

Start here.

iPhone Repair and Service - Apple Support

Parts fail, it's a fact of life. Your battery has failed and needs replacing.

Click on the link that sberman provided you and get your phone repaired.