What OS ? If its NT, try creating another test service name to the database you wanted and see the test is successful and you could connect to it. If you could then there is something goofy, try replacing the host name with the ipaddress of hostname and try.

I Don't think, tnsnames path which was working yesterday is something to do with registry. DOn't play with registry unless otherwise you have to.

As sambavan suggested, try stopping your networking components(Listener, dbsnmp agent, data gatherer) and restart 'em and try connecting to database.