that looks like it is in the 'off' position. if you twist it should spring into the tyre. there may be a button to press underneath. some have them, some just need a hard push to overcome the spring tension. i'm no expert as it is a long time since i have had a tyre dynamo. hth.
The dynamo is in the 'off' position. To engage, push down on the little button bit (on the flat surface below and to the right of the roller) and the whole body will spring inwards and twist (the top will swing in and down) to rest on the tyre.
To disengage, pull back (gently) holding the dynamo body below the drive roller and it will click into the 'off' position.