Hello James,
Thanks for reaching out to Apple Support Communities. It sounds like you're having an issue with loading the Stocks app. We're happy to help.
Does this issue occur on Wi-Fi, cellular data, or both? If both, have you tested when only using one or the other? Also, we recommend ensuring the Stocks app is turned on for cellular data use in your cellular settings. You can navigate to Settings > Cellular or Settings > Mobile Data, to check. If needed, the article here can also help: Check the cellular data usage on your iPhone and iPad
If the issue continues, test opening the app from your 'Today View' widgets screen. Swipe right on the Home screen to access the section. The article here can also help: Use widgets on your iPhone and iPod touch
Additionally, you'll want to be sure there aren't any restrictions or blocks set up: Use parental controls on your child's iPhone, iPad, and iPod touch
Cheers!