Windows 10 may have officially launched on July 29, but many preview versions of the OS were released beforehand as part of the Windows Insider Program. On Thursday, October 15. many of those builds will expire, which will keep those PCs that have them installed from booting up.
Specifically, if you still have Windows 10 Insider preview build 9926 to 10166 installed on your PC, you will run into this issue tomorrow. Microsoft says:
"After this date, you will need to perform a boot from media installation of Windows 10 RTM and opt-in to the Windows Insider Program to continue to get new Preview builds. Because your Preview build has already expired, it is no longer activated. As a result, you'll be required to use the Windows 10 default product key to upgrade, and afterwards you will not be activated."
The company's support forum post has a detailed breakdown of how to boot up your PC and install a newer version of the Windows 10 Insider preview if you let your older Insider build expire on Thursday. Microsoft just released a new 10565 preview version of Windows 10 this week.