The instructions for swapping hotends is pretty specific about turning the printer off before attempting the swap.
https://wiki.bambulab.com/en/p1/maintenance/complete-hot-end-assembly
That being said, try reseating the connectors again (with the printer power off).
Make sure to press hard on the connectors. Also, I have found that it works better if you put the wires under the clip before you reconnect the connectors or replace the screws.
If that still doesn’t work, try reconnecting the original hotend you pulled out when things were still working.