The Windows RT 8.1 update for devices such as Microsoft's Surface RT has been removed from the Windows Store temporarily, after a "situation" prevented a "limited number of users" from being able to upgrade successfully.
The problem appears to be that the update is damaging certain boot data, causing affected machines to blue screen on startup. The issue is recoverable if you've created a recovery USB key (or have access to a machine that can create one), but Microsoft currently appears to have no easy way to create a suitable USB key from non-ARM machines.
To call this embarrassing for Microsoft is something of an understatement. While x86 PCs have extraordinary diversity, in terms of hardware, software, and drivers—all things that can prevent straightforward upgrading—the Windows RT devices are extremely limited in this regard. Upgrading Windows RT tablets should be absolutely bulletproof. It's very disappointing that it isn't.