Well I think this is a bit trouble. You said most of your tables cant be queried then seems like it´s the main datafile that is missing and you said it´s not anywhere, so it was deleted accidently? If it was deleted then you will have to do a recovery, it can be a few situations, I doubt I can help remotely.
Do you have backups at all?
Your company should hire a DBA, really. Oracle is pretty complex since it offers so many options, features, and depending on situation you do a specific thing.
If the boss doesnt want to hire a DBA I would suggest using SQL Server.