Firstly, you'll need to report this in the app:
Tap "Support"in the app
Tap "A vehicle is in my space"
Enter that vehicle's registration number
The app will re-allocate you another space, if one is available. If no other space is available, your parking session will be ended without charge.
If your location service's are turned on in your phone, the Parkable app can show you other car parks nearby that may have spaces available.