here is what I would try...

1- getting into the database

sqlplus /nolog
conn sys as sysdba

If you get into the database, then...

2- Solving the issue

Here you can do whatever you want like:
a) truncate sys.aud$ table... or
b) resize a system datafile to a larger size... or
c) just add a datafile to your system tablespace.

Hope this helps.