Microsoft rolled out the Lumia Cyan update recently for all existing handsets running on Windows Phone 8. However, users who are on the latest version of Windows Phone 8.1 Developer Preview are raising major concerns. In particular, the problem seems to occur for all those who use BitLocker to encrypt data on the disk. For these users, the phone becomes unusable after the upgrade.
Microsoft has issued an official statement, encouraging users to downgrade from Windows Phone 8.1 Preview for Developers to the commercial software before downloading the Lumia Cyan update. The company has also suspended the preview program and the 12400 build on a temporary basis.
Microsoft stated: "We are currently investigating the issue. We have paused delivery of the update to customers using the Preview for Developers, and will resume once a fix is in place. Customers whose phones have been affected should use the Nokia Software Recovery Tool to return your Lumia to a normal commercial software state before downloading the Lumia Cyan update." This issue affects those who have BitLocker enforcement on a company phone, and likely doesn't affect a regular user.
The best option would be to wait until Microsoft can fix the issue and continue the rollout. Rolling back to normal commercial software state using the Nokia Software Recovery Tool can fix the issue but it will also wipe any data currently held on the handset.