khussain has given you solution, here is explanation.
What has happened here is that you have specified private rbs's in your database/init.ora (specific to that instance) and then subsequently dropped one of them. When the database tries to open, it realises that your rbs is not there and error is produced.
You can stop this from reoccuring by.
1. changing your init.ora every time you reorganise your private rbs's/add new private rbs's.
2. create public rbs's that do not need to be specified in init.ora by using 'public' keyword.
create PUBLIC rollback segment rbs01
storage (blah blah blah)
Once you have eliminated all of the impossible,
whatever remains however improbable,
must be true.