Can you see the block when previewing? On the right hand side, choose the context menu(the one with three dots), then choose Preview.
The thumbnail can be generated only if it can be rendered. Some common reasons that makes it hidden:
- Not visible for any user/group. Check its permission.
- Only visible for some specific groups. Check Audience and Permission.
- Not active in any channel, or the active date is either in future or in the past. Check its channel setting.