My MacBook can’t be started now. I have an appointment at Apple for repair. Is it too late to back it up

Hello, maureen21.

No, unfortunately. It is too late to back it up. If you cannot boot your Mac you cannot run a backup.

You're lucky that "most of your data" is in the cloud. Those files won't be lost.

IF you could boot your macBook into Target Disk mode, (restart while holding the T key) then you could possibly do a copy or file transfers to another Mac that way. It kinda depends on the cause of the startup failure.

Transfer files between two Macs using target disk mode - Apple Support:

https://support.apple.com/guide/mac-help/transfer-files-mac-computers-target-disk-mode-mchlp1443/mac

Once you get your laptop repaired, then make sure to start frequent and regular backups since iCloud is not a backup.

Older non-USB-C Macs have removable drives, but many of them require a special USB enclosure from OWC to be able to access the data from another Mac. For any USB-C Mac you can tell Apple that you need data recovery services so they can assist you with sending the Logic Board (or the computer) to a professional data recovery service to attempt to recover the data. With a 2018+ Mac with the T2 security chip the process is much more difficult or perhaps even impossible for recovering data. If you have a MBPro 2016-2017 model, then an AASP (not sure about an Apple Store) can attempt to recover data from the SSD by using a special device which connects directly to the bad Logic Board (this service will cost extra). The important part is to let the technician know about your data recovery needs before the repair is performed. Once the repair is performed you may lose any ability to recover any data.