If you are certain you left and re-entered the car park, you need to look for any evidence you can provide that will prove this is the case. You could look at dashcam footage (if you have one), receipts for purchases you have made elsewhere or even parking tickets for other locations.

You can also request that the parking company review their own CCTV footage - it is possible they use Automatic Number Plate Recognition (ANPR) to register vehicles entering and leaving the car park, but that for some reason your exit was not captured. Use the appeals process as recommended and include all evidence you can that backs up your claim.