Can't turn off calculator

[lnex_x]

Shift + On no longer turns my HP Prime off. Now when I had to use it this evening, I found the entire keyboard didn't work except for Shift and ALPHA, but I fixed that by reverting to Firmware version 20181016; I had been using the latest version. However, I still can't turn it off. I tried following this document: http://www.hp-prime.de/files/composite_f...y_mode.pdf to reset it via recovery mode (clearing memory doesn't help), but the screen goes black and nothing comes up despite waiting for a full minute. I've also tried removing the battery for an hour.

Please help, thank you.