I just bought a new iPhone 12.
The screen on my current iPhone has stopped working.
On the new phone, I've downloaded WhatsApp, but all the messages I had on the other phone ( 3years worth) have all disappeared.
I don't think they were backed up to iCloud.
Is there any hope of getting those back?
The SIM card is still in the old phone. I haven't transferred it yet.
Have you verified the phone number on a new phone?
If yes, then you may not able to access old phone messages.
For now, verify your number on your old phone again, there will be back up in your phone storage as well, that will be recovered.
Then take a backup on Google Drive or iCloud and then transfer to new phone.
Hope it will work.