My iPhone app crashes at startup?

I downloaded the Plant vs. Zombies app yesterday and when I tried to open it today, at startup it asked for my Apple ID and password. After i entered my ID and pass, it crashed.

Try closing a few apps, you maybe running low on the old RAM, to do this you will have to double press your home button then hold on one icon, then press the minus button to close the app, generally your apps will save, but paused games will quit and you may lose your progress, if you are jailbroken, these kinda things happen often: P.

Normally PvZ does not require a password.

next time before you open big-sized programs, shut others down first.
e.g.safari webpages, etc.

And, deactivate your internet connection like Wi-Fi.