Welcome to Apple Support Communities. As I'm understanding it, every time you open Snapchat your phone will freeze and immediately restart and you have already tried deleting the app and reinstalling it. Let's go ahead and try the following steps (even if you have already tried some of them) in order from start to finish to see if that fixes it.
Check out this article with steps to Force an app to close on your iPhone, iPad, or iPod touch . This will allow you to close the app you're having issues with.
Another article you can check out is this one for If an app you installed unexpectedly quits, stops responding, or won’t open .
Next, check out this article with steps to Restart your iPhone, iPad, or iPod touch .
If this is only happening with this particular app and nothing else, then I would also recommend possibly reaching out to the app developer themselves as well to report this issue and see if they can provide some additional assistance.
Please feel free to keep us posted on your progress, or if you have any further questions. Otherwise, wishing you a wonderful week.
Happy Holidays!