Hi @ducapedia
Don't know about your particular model but with a lot of smartwatches the display is an LCD display which means that it has a backlight.
With the watch turned on, try shining a torch (flashlight) at an angle close to the screen and check for any images on the screen. They will be ''very faint'' if they are there so trying this in a darkened room may help to see them.
''If there are images to be seen,'' then it is a backlight problem. Either a problem in the screen or a problem with the backlight power circuit on the systemboard.
Without a schematic of the systemboard (I can't find one) it is hard to know but some may have an onboard smd type (surface mount device) fuse in the backlight power supply circuit which may have blown or perhaps it could just be an IC that is turned on to supply the power to the backlights.
''If there are no images to be seen,'' then it could be that the swollen battery caused more damage (electrical?) than what is seen, either in the screen or on the systemboard or both.
Hopefully you had the new battery disconnected before you removed/replaced the display cable as it is always advisable to disconnect the battery before working in electronic devices and then reconnecting them last thing after you're finished so as to avoid causing any problems when working on them. The trouble with a lot of electronic devices these days is that there is always power on the board somewhere, even if it is turned off. The power button (or On button) is ''not'' a power isolating button. Its function is to send a signal to turn on or off. Think of the device as being in an extremely low power state and not disconnected from the power.