Your TNSNAMES.ORA file should be in the [drive]:\[*oracle]\ora81\network\ADMIN\ directory. *ORACLE is the directory I loading the files onto, replace this with the file directory you set your copy of ORA into.
It needs to be double checked for the proper desc and protocol and IP mapping information in order to mount and open the db over the network.
My file is marked, 'Generated by Oracle configuration tools.' and is only 2k in size. It is normally set up as part of your install
I accidently crushed this file when I installed Ora 188.8.131.52.0 local to my system and overwrote our network generated copy of the tnsname.ora file. As for rebuilding it, I can only hope that you can find the