Switch cannot boot past Hekate after replacing fan

Sounds like your SD card was being read as your device was being disassembled. I use SX OS but same story minor SD issues cause the console to hang after the Nintendo logo on any firmware. It's always stressful taking the card out for me I've done it a few times to myself. Make a backup and format and see what happens. Your issue is absolutely software based if you can boot to Hekate and flash eMMC.

 

Format the card itself or put another card with just OS and see if the behavior changes. I get this behavior sometimes and formatting my card has brought me back from it in the past when flashing eMMC with a backup did not. The card is being read since Hekate is booting from it, I'm thinking something else on the card is causing an issue. Our situations may be different as my Switch will enter RCM on its own and try to boot. Since the battery can't be disconnected until after the SD reader that gets problematic for repair and has caused me this issue before.