I know it sounds simple, but it's often overlooked. Could you try restarting your device? If that doesn't help, try a quick clean reinstall of the app. Just follow these steps.
If you're still having trouble, could you let me know the exact version of Spotify and the exact version of the operating system you're running?
I'd also need a bit more info: What happens when you try to log in?
I'll see what I can suggest next! Make sure to ask any other questions, I'll be happy to help.
Cheers, Jose 🙂