Q: backup taken now phone can be restored

Hello rajatbhurat,

If your iPhone is low on space that may cause other issues. With a good backup you could erase and restore from your backup but first you may want to free up space to see if that helps.

Check your storage on your iPhone, iPad, and iPod touch - Apple Support

For Whatsapp hanging, as well as freeing up space, please check out this article,

If an app you installed unexpectedly quits, stops responding, or won’t open - Apple Support

If none of those help, try the restore.

Restore your device from an iCloud or iTunes backup - Apple Support

Best regards.