I recommend noting down your player ID before you try to do any recovery method below.
1. Make sure to log in to Game Center so a copy of your game progress is saved.
2. If you turn off your WiFi and internet data, could the game be started up to the main menu? if so, please register your email to get 100 free gems if you have not already done so as this would tie the PVZ2 account to your email and also enable you to try recovering your progress on another gaming device. If the game is unable to startup to the main menu, then there is little you can do except to try a delete and re-install, normally IOS will re-download the game and recover your progress from the Game Center.
3. If the above method fails, you last chance is to contact EA HELP CENTER by clicking the link at the bottom of this page and open a 'Missing Content' ticket with your player ID and get further assistance from the advisor.