Chosen Solution
So about 5 months ago I left my MacBook Air 2015 13’’ in a hot car (by accident) for a day mid-summer. It shows on my dell monitor when I plug it into it via hdmi. I can only see the display if I shine a light through the apple logo or if the display is in direct sunlight.
It may be the case that the display cable has become slightly disconnected so the backlight power is not being communicated properly to the display. Try removing the bottom plate, disconnecting the battery, then disconnecting and reconnecting the cable shown in Step 20 of this guide. Then reconnect the battery, put the bottom plate on and power it on. If the issue still occurs after reseating the cable you may have some internal damage, either to the display or to the logic board. You can also try a PRAM reset, but it’s pretty unlikely that this will help. To perform a PRAM reset, hold the option, command, P, and R keys while the system is powering on. Continue to hold them until the system reboots on its own and you hear a chime. If neither of these things work, you’ll need to take it to a shop where they can try out a separate display, to determine whether or not it’s a board level failure or a display level failure. Hope this helps!
