When I switched on my laptop this morning, without any input from me, Windows 10 decided to download and reinstall itself from scratch. There was a message saying there was an update and a restart was needed. All my files and programmes are still there.
After Windows installed-there was a notification that an update was installed, but there is no record of one.
Was there a problem in the booting up process which Windows detected? Is this a new feature of Windows 10? Is it anything to get concerned about?
Thanks in advance.

I wonder if there wasn't a large update that was required - maybe for the UK??? I haven't gotten anything and as far as I know the next big update to the entire Operating System isn't supposed to be out until this summer sometime.

Updates in Windows 10 are a little different depending of the version you have. If it's Windows 10 Home, you don't have any choice about updates - you just get them. If you have the Pro or Enterprise versions, you can set it to review updates first.

Windows 10 obviously did something it needed to, so I wouldn't worry about it.

Are you sure it reinstalled its whole self and not just an update?

I wonder if it wasn't this cumulative update from April 12? It was a fairly large update, but certainly didn't download and reinstall the OS.

https://support.microsoft.com/en-us/kb/3147458

The 'update' screen had the words 'Upgrading Windows' a white circle with the percentage showing. At the bottom 'copying files, installing features and others and configuring settings'. Definately a complete OS installation in my book! The whole process took around 2 hours.
I have looked in Event Viewer and there is nothing before today's date (24th April). One of life's little mysteries.

Apparently it is "business as usual" with Windows 10.
https://www.reddit.com/r/Windows10/comments/3vpazx/wtf_windows_10_randomly_reinstalled_itself/

Thank you, Jenny. Perhaps a warning from Microsoft would be good.

Oh... wait. You didn't already have Windows 10 installed! I thought you were saying that Windows 10 was already installed and this happened.

There were ways to stop this from happening, but if you didn't do them, then as Jenny says, Microsoft is really pushing this upgrade. If you decide you don't like it, you can go back to your previous OS within 30 days. Then you will need to do a couple of things in your updates to be sure Windows 10 doesn't install itself again.

I think Upsydaisy already had Windows 10 installed.
The problem was it re-installed the whole OS to a newer version of Windows 10, instead of just adding an update. That's what happened to the guy in my link.

Or if it was "just" an update, it was a massive one that appeared to be the same as a complete reinstall, and erased any trace of the previous Windows 10 install (and subsequent updates) in the Event Viewer.

Spot on Jenny. I checked my windows.old file and it contained the previous version of windows 10 plus all my files and programmes which were kept and transferred (if that's the right word) over to the newer version 10586. The first version in July 2015 was 10166

Well, I was obviously confused! Windows 10 definitely has a mind of its own when it comes to updates - especially if you have the Windows 10 Home version.

My current build is 10586.164. I assume you have the 1511 version - that's the most recent release from November, 2015.

And it looks like we will have more to look forward to - or not - in the next few months. Windows Insiders did get a build update last week previewing the next big Anniversary Update.

Preview of Anniversary Update

Thanks for the link Draclvr. Microsoft is certainly moving with the times!!