R6 Mark ii not booting

[TimSe]

Earlier today I was shooting and the original battery started to warn (red blinking battery icon) so I popped the battery and replaced it with a somewhat older LP-E6N. Since then the camera is completely dead. 

When I insert a (charged) battery and hold my ear against the body I can hear 2 sounds: something that sounds like the IBS starting up (mechanical "klunk" like sound) and a surring sound like a fan. The screen stays black and the camera does not respond to any buttons. 

Any tips?

[Tronhard]

Hi Tim and welcome to the forum:
The R6 should work with the earlier versions of the battery. 
Were you shooting video or still images?
When you say "so I popped the battery and replaced it", did you turn the camera off to do so?