Check your connection and read any error or access message shown by the app. The message may tell you whether the problem relates to your plan, credits or the request itself.
If no generation is still running, try again after reopening the app. For image requests, make sure the description clearly explains what you want.
If it keeps failing, send support the error, the model or tool you selected, and when the problem occurred. Include the prompt only if it does not contain private information.