My minecraft PE won't open?

It worked perfectly the first day but now it won't start every time I try to open the app it crashes before the game menu is on the screen I don't know if it's important but a friend downloaded the app in his account for me in my iPhone 4

Turn the iphone off and then turn it back on. It usually closes all of the apps. Or reset the all settings, it helps too. If not then unistall and install the app again. If you uninstall the app, dont remove it from the iphone, go to itunes on your computer and unmark the app in the applist, and then sync, and then mark the app again, and sync.

But i dont want to delete it

I dont want to delete it either

I don't won't to delete it because I have a lot of things in it