How much free space is available on your device? You can check in Settings/General/iPad Storage while that application is running with the file in question. Photoshop requires 2GB to launch but if you are at the low end of available space and you open or create a large file it can use up disk space so that there is not enough for you to perform tasks. You would need to free up additional space or use smaller files or fewer layer to prevent hitting that state.