Hello Antonio,
1-Normally you can find the spfile and pfile (mostly initSID.ora) under this path: /oracle/SID/102_64/dbs/ .
2-You can use primitive Oracle parameter check script which is attached to that note.
My recommendation here is just takin a backup of your spfile and then reset the parameter:
For example:
cp -p spfileSID.ora spfileSID.oraORIG
sqlplus > alter system OPTIMIZER_FEATURES_ENABLE scope=spfile sid='*';
If that OPTIMIZER_FEATURES_ENABLE parameter is an online one you won't need a restart and if so you can use that command:
sqlplus > alter system OPTIMIZER_FEATURES_ENABLE scope=both sid='*';
"Both" forces updating both spfile and real-time (in-run) value of the parameter.
Regards,
Serhat