Photos won’t load on iPad

This might happen, if you disable the Photo Option in the settings for iCloud on your device, for example.

As long as the photos are still saved in iCloud/Photos, you can always download them again.

A backup of your data might be a good idea, in case they will get deleted in iCloud as well somehow.

Archive or make copies of the information you store in iCloud - Apple Support

Your iPad is connected to Wi-Fi and being charged at the same time?

Depending on the number of photos you want to sync, this may take a while and should not be interrupted. Otherwise the process will start again from the beginning.

So, maybe let it stay connected during the night and check the next morning for any progress.

If the iPad is charging, and you connect it to Wi-Fi and iCloud to sync, the Wi-Fi connection will not be terminated when the display enters standby mode.

Only if your device is not connected to the charger, Wi-Fi will be disabled during standby, to save battery power.

So you should be good, if you start the sync process and let it be connected to the charger over nigh. No need to leave the display always on during that time.