Have your lawyer contact the phone company then.
Nothing we can do for you here.
If you have lost your messages, you will need to contact your service provider and request them to resend them to you. Or in this case, have your lawyer do it.
Carriers keep copies of all messages sent and received on their servers for legal reasons. Say the police believe the phone is being used to take orders and traffic drugs. They would obtain a warrant, and the phone company would issue copies of all the messages.
This is only in place for legal reasons, so you need not worry about someone reading everything you say in your messages.
Some carriers have the messages available for their users online, like Bell Mobility in Canada for example. Others, you will need to get a hold of them and request this service. They may charge a small fee.