This one can be a doozy and when I was recently on a sales trip doing demos, this happened to me and it took me 45 minutes to do the factory reset and re-install of the app over my phone's cell connection.
Here is the link to HTC's list of instructions: https://forum.vive.com/topic/6229-htc-vive-focus-how-to-factory-reset/
You can refer the steps for factory reset and then see the menu of enter bootloader (A 2D screen)
1.) Make sure that the Focus is powered off.
2.) Hold the volume up/down keys at the same time and at the same time press the power button.
3.) You should see a bootup menu.
4.) Press the volume buttons to cycle through the menu (go to Reboot to Download mode).
5.) Use the power key to confirm your selection.
6.) Inside the headset you should see an option for "Factory Reset".
7.) Select that option, by using the volume keys.
8.) Press power key to confirm.
9.) If successful you should see the Vive logo and a percentage counter. Within 30-60 seconds you will be taken to setup screen with Chinese text and an English VO called Vive Dimensions.