You will lose everything on your phone if you restore your phone as new. It's like buying a new phone, so all your information is removed from the phone (such as SMS messages, app settings, wifi password, phone password, call history). Essentially, everything will get deleted during the restore, and it will only reload what is on your
computer when you sync the first time.
That's why you should always have a working backup of your phone, before doing a restore.
With that in mind, you can restore your iPhone, without doing a completely "new" restore. When doing a restore you can choose "Restore from the backup of:" your iphones backup copy. When you restore your iphone from the backup, you should retain all your previous settings, text messages, apps, etc., and will just need to reset your iPhone password (if you use one to lock your phone).