Steam has a native feature that allows you to recenter and orientate yourself in VR.
If you find you've gone off centre while playing BoxVR, you'll just need to take the following steps:
Go to the SteamVR dashboard
In the bottom left hand corner, you'll see an icon of a person with an arrow around them. Click this icon.
This will start a ~3 second countdown timer at which point your position and looking direction will be used to re-define the centre and forward directions of your play space.