First check your environment and make sure that everything is setup correctly path etc. Second check the registry and see if the Oracle hive has been corrupted. You might be able to compare it to another machine if you have Personal Oracle installed elsewhere. Finally post your listener.ora, sqlnet.ora and tnsnames.ora files. As well as go to lsnrctl do a status and post that. It could be any number of things. But without a clear error message and the above files it is hard to tell.