It's that time again, the time for Microsoft to end support for a version of Windows 10. According to the Windows lifecycle fact sheet, support for Windows 10 for PCs version 1511 will be ending tomorrow, October 10.
Version 1511 was the first update to Windows 10, so this is only the second time that this has happened. The first was on May 9, when the original version of the OS, now known as 1507, was removed from support. The company has continued to provide cumulative updates though, despite that, for business customers that have different licensing agreements than consumers have.
Naturally, your PC should have been moved to at least the Anniversary Update (version 1607) or the Creators Update (version 1703) by now. If you haven't been, you can manually upgrade your machine to a newer version here by clicking 'Update now'.
Note that this change only applies to the PC version of the OS. Windows 10 Mobile version 1511 is still slated to be supported until January 8, 2018, despite not having received any updates in quite some time.