Start SQL services after IP change

Hello

We have two SQL instances in the same computer: default (MSDE) and named instances (SQL 2008).

We changed the IP address a few days ago, and suddenly we cannot start the SQL services in either instance.

I think in the past when I did a select @@servername, the result was

- for the default instance, it was the IP

- for the name instance, it was the IP\instance_name

Is there a work around other than changing it back to the original IP or re-installing SQL server?

Thanks

Carbond.