This afternoon my HP 28C would no longer work!

(Had worked properly ever since I bought it 23 year ago, and still did so this morning.)

When I tried to switch it on, there were only two horizontal lines on the display.
http://www.stadler-christoph.de/P1030754.jpg

And I could no longer turn it off.

I tried the self tests, and they would still work, and the display was fine during these tests.

But afterwards there still were only these bars.

I replaced the batteries for a new set (gladly our company still has them in stock).
But this didn't help.

After putting the calculator away for a few hours I tried it again.
And now it works again properly - for the time being ...

Any idea what could be the problem ?