Why won't my Minecraft PE open?

I got this app a few weeks ago and it worked fine. Today I tried opening it and it just immediately went back to the home screen. I'm afraid if I delete it and redownload it, will it delete all my worlds?

jessica 07.07.2015

What device are you using? Apple or Android? If you are using Apple, I recommend these:

1. Double-tap the home button and quit the application.
2. Check for any
a. Minecraft PE Updates
b. Software Updates
3. Do a hard reboot by holding down the Power button and the Home button for about 10 seconds, then swipe "Power Off". To wake the device back up, hold down the power button until the Apple logo appears on the screen.

For Android, I'm a real noob. If possible, do the above.

Kylee 07.07.2015