As The Register reports, some users are writing that Anniversary Update rendered their PCs unusable. The update installs and after reboot nothing happens. Some folks managed to get Windows 10 Anniversary Update running by disabling Secure Boot or by using certain workarounds, but there's not always a solution:
Microsoft, for its part, has advised those in the thread to do a clean install of Windows and pointed those afflicted to USB media creation tools in order to facilitate that action.
Some users, however, have reported that even that measure isn't working. One method that restores PCs to a useful state is to roll back to a previous version of Windows, which probably isn't what users want seeing as they're trying to upgrade! And no use at all for those who may have done complete wipes of their machines.