Q: Why can't my macbook air find my iPhone 6 when connected via lightning cable

shut down and disconnect all external devices then reboot into safe mode

Try safe mode if your Mac doesn't finish starting up - Apple Support

when you log in connect the cable to the phone, unlock the phone and connect the USB end directly to the computer, and not through a usb hub

Open iTunes and your device should be shown, or you may be asked to trust the computer on the device.

if the device connects reboot normally

That is not the soution

Chances are that the solution that JimmyCMPIT suggested will work

Another probably solution is to connect the Phone and open Photos and restart the Phone - this ften will resolve this - if not rebooting the Mac and restarting the Phone while it is connected should do it

LN

Macbook can find iPhone 6 in iTunes, but not in iPhoto

iPhoto will only show the iPhone, if it is seeing new photos on the iPhone.

It may not be able to see new photos, if iCloud Photo Library has been enabled in the iPhone.  What are your settings on the iPhone?  Is iCloud Photo Library enabled with "Optimize Storage"?  Then the original image files may be in iCloud and not on the iPhone and you cannot use a USB connection to download them.