there is other post running since last few days with title "connection problem to database instance" by midora for the same error. Please read the whole post and see if you are missing any thing in configuration.
Try hard to get what you like OR you will be forced to like what you get.
In case really! tnsnames.ora file was crashed. If u Know Host or IP address andPort and SID then You can create a new tnsnames.ora file with Host name or IP address and SID and port are mandatory information. Your problem will resolved. Good Luck
after i patch upgrade from 10.2.0.1 to 10.2.0.4 i get this error when connecting to sqlplus
i think that the things is not simple.
1, you patch your database in host server and local pl/sql connection does not require tnsname.ora file. but remote connection need it.
2, i suppose that you should check your listener information. in one case i have met the err, because the size of listener.log had reached 2GB on Linux system.
2G of listener.log! Need to check that. Also may be your new database is not registered to this listener, run lsnrctl, if it doesn't show any database registered to it, trying create new listener using netmgr and restart database. It should help.