My Macbook air is not starting at all and I would like to know,how data that has not been synced or backed up in iCloud?

You'll have to get it repaired if it doesn't boot at all in order to recover your data.

Genius Bar Reservation and Apple Support Options - Apple

If it does boot, but displays a question mark folder for instance, see If your Mac starts up to a question mark - Apple Support

You can also check if Use target disk mode to move files to another computer - Apple Support works

An 'erase and reinstall' (with backups, from external archive/hard drives) could work.

• How to reinstall macOS from macOS Recovery - Apple Support

https://support.apple.com/en-us/HT204904

The A1466 number applies to at least three build models of 13-inch MB/Air.

So to correctly identify your specific build model year + specs, use Lookup.

• Lookup Mac Specs by Serial Number, Order, Model & EMC Number, Model ID | everymac

https://everymac.com/ultimate-mac-lookup/

• Mac Detailed Specs and Parts List by Serial Number (lookup)

https://www.powerbookmedic.com/identify-mac-serial.php

A1466 Model applies to the Mid 2012, the Mid 2013, and the Early 2014 MacBookAir.

And remained in use through some of those shipped 13-inch MB/Air in 2017.

Should you have backups from your MacBook/Air storage drive, [incl. Time Machine

on external USB drive] then your portable Mac may be candidate for 'send-in' repair.

• Apple Repair and Repair Status Check - Apple Support

https://support.apple.com/repair

As part of the send-in service, authorized repairer service could send a return box.

Identification helps in several ways; avoids misinformation and guesswork too.

Good luck & happy trails!🌻🐝

It depends on the problem, and on the model of MBA you have. It might be possible to transfer the HDD/SSD to an external enclosure and copy off the data. But, more recent Mac notebooks have the storage soldered to the logic board so it's not easy (and may not be possible) to recover the data.

The moral of the story is to always have a backup. Preferably more than one, and ideally stored in more than one location.