If you still have your backup phrase
Open the app and go to : Add Wallet > Import Wallet > Wallet Recovery Phrase. Write down your 12 words backup to restore your funds.
If you have a file or text backup, go to Add Wallet > Import Wallet > File/Text Backup in the app and upload your file/text backup.
If you do not have your backup phrase
If you do not have your backup, then unfortunately you have lost your bitcoin. For security, 12 word backup phrases are not derived from or stored on BitPay servers, so BitPay cannot recover funds for you.