When this occurs, the progress bar might be moving very slowly or seem like it isn't moving.
Let the device complete the update, restore, or erase process.
The progress bar shows installation progress. The amount of time depends on the number of files on the device and whether you're erasing, updating, or upgrading your iOS. This process can take as little as a minute if your device has little or no data or if you're erasing the device. If your device contains a large number of files, the process can take from several minutes to an hour.
If your device appears in iTunes, you can configure sync options, and you can sync content, the process is complete. Leave your device connected to iTunes and wait for the migration process to finish.
If you're updating over the air, connect your device to a power source so that it won't run out of power during this process. If your device runs out of power, connect it to a power source and turn on the device to let the update or restore complete.
In some cases, you may see the progress bar complete, then start again.
If the update or restore never completes or your device is unresponsive, force your device to restart.