MacBook air with touch id. Not turning on. What do i do?

That means it cannot find a valid disk to boot from. It would normally boot from your internal disk. If it cannot, the disk may have failed, or it might simply need repairs. Hopefully the second is what has happened.

This Apple Support article https://support.apple.com/en-us/HT204323 has instructions for doing those repairs yourself. It is important to follow the instructions exactly and carefully.

The section of the article that I think best applies to you is this section:

"IF YOUR MAC DOESN'T START UP

If your Mac starts up to a flashing question mark, but then doesn't continue starting up, try these steps ..."

The support article then explains how to boot into "recovery mode" and run diagnostics/repairs that will hopefully get your computer back to normal.

No, it's a laptop which should start up and run off its internal battery. Unless the battery has no charge left, in which case it won't start unless plugged in. If it won't start when plugged in, that could be due to any number of internal hardware problems (including the battery) or it could be due to a faulty power supply not charging the laptop battery or transferring power properly.

Does it work ok when plugged in?