Help on manual reinstall Sql Service

[Database Administrator]

I had to recover an old server configuration.
Now I can't see the sql server service in services snap-in
The sql is installed because I can see it in the "add or remove program",
but I can't reinstall it because it say that there is a default istance yet.
I can't uninstall because I get an error.

Is there any way to reconfigure the service to start it again?
Thanks.

Roberto