Bangle (original) won't restart after installing app

That's extremely strange - but to update the firmware, the instructions you mentioned at https://www.espruino.com/Bangle.js#firmware-updates say:

• Long-press BTN1 + BTN2 for about 6 seconds until the screen goes blank
  
• Release BTN2
  
• Release BTN1 a moment later while ==== is going across the screen
  
• The watch should now be in Dfu mode
  
  

I think you missed those steps. When in DFU mode the display should show DFU START in the blocky font, and I bet yours had booted back up to a clock face or the Bangle.js logo?