you need to copy release.config to web.config and adopt SQL connect string and maybe object qualifier, so it meets the setup of your database. If you added modules or providers, that needed modifications to you web.config, you need to insert these tweaks as well.

Now it remains, that you are missing the encryption keys. Can you remember, with which DNN version you started your site? AFAIR some of the earlier ones had fixed values, so try to get them from the web.config of your initial installation version, if it works and you can login as host - you survived.

If there is no web.config or the keys are missing, you will need direct database access, to reset the host account password. call your site and create a new user (remember the password). Goto the database, table aspnet_Membership and copy password and passwordSalt from the new user into the appropriate fields of the host user. Login and set password of admin. All other users will have to retrieve their passwords and might need to change it back to the original values.

If you have a host, are you sure, he did not make a backup?

I do not know the connnect string for your database, only your host knows this. He should know as well, if he uses an object qualifier in the database.

if you really started with 3.0.12, the encryption keys should have been   

<add key="MachineValidationKey" value="F9D1A2D3E1D3E2F7B3D9F90FF3965ABDAC304902" />
  <add key="MachineDecryptionKey" value="F9D1A2D3E1D3E2F7B3D9F90FF3965ABDAC304902F8D923AC" />

Try to log in with your account/password. If it works, you got the correct keys.

Good luck!