You may find some contacts are missing from your phone after updating to Sense 6. This may affect the following devices:

• HTC One (M7)
• HTC One Mini
• HTC One Max
• HTC Desire 601

If the missing contacts are tied to an account such as Google, Exchange, etc, follow these steps:

1. Remove the accounts that contain contacts by navigating to Settings > Accounts & sync > [desired account] > three-dots menu and tapping Remove.
2. Clear data for the contacts storage by navigating to Settings > Apps > ALL (tab) > Contacts Storage and tapping Clear data.
3. Re-add the desired accounts by navigating to Settings > Accounts & sync > + icon > and following the on-screen steps to set up the accounts,
4. Toggle Auto-sync off then back on and ensure all accounts sync properly. 

If the missing contacts are stored on a computer app such as Outlook, use HTC Sync Manager to re-sync the contacts to the device.

If the missing contacts that were on your phone but were not backed up, you will not be able to recover them.