Hi Robin,
Sorry to hear that some files are missing.
Sometimes files get deleted by mistake and go to the online recycle bin, where they remain for only one month before being permanently deleted. So checking the recycle bin monthly is highly recommended.
I'm sure you know you need to keep an active Microsoft 365 subscription.
I highly recommend backing up your files to other media, not just OneDrive. OneDrive is basically a file-syncing service, and mistakes like these will multiply across all your devices.
Here's the Contact Microsoft Support link mentioned in the AI answer: https://support.microsoft.com/en-us/contactus
Please see the AI answer for more information. I hope you can find your missing files.
Craig