HP-12C 1985 vintage repair

[rprosperi]

Does it flash "running" a few times prior to showing that display? If so, then it's running it's self-test, as this is the screen shown when it completes successfully. This is normally launched by holding down the [X] key, then press and release [ON] and then release [X]. Is it possible your [X] key is jammed down?

Also, try this: Remove your batteries and then short the 2 battery terminals together; you can use a small piece of wire or even a pair of scissors - any metal conductor. While shorting the terminals, press and release the [ON] key. Then re-insert the batteries. Amazingly this simple procedure sometimes succeeds in clearing a machine that is locked-up in some confused state.

Good luck