It does not work..!

I assume by ‘shift’ you mean the ‘2nd’ button?

This issue quite perplexed me, I have no clue how you got to this state, but here’s how you could get your calculator back to normal.

The first thing I’d try is to:

1. remove all the batteries, including the backup battery (there’s a guide on how to remove it here.)
2. Hold the ‘on’ button for 10 seconds
3. Put all the batteries back in (starting with the backup battery).

If that doesn’t work, try it again but, as Aaron suggested, with fresh batteries (including a fresh backup battery)

If that still doesn’t work, you’ve somehow broke something and it may not be a software issue. However, there are a one more thing you can try, an OS reinstall. Now that may sound scary, but it’s not quite as difficult as you’d expect. It’s hard to give you accurate steps in text so I’ll refer you to a video tutorial.