To further expand on my comment, the
sys.aud$ table is stored, by default, in the SYSTEM tablespace. When the SYSTEM tablespace fills, your database halts.
There are a few options available to you to manage this, but these will depend on what version of 10g you have installed. As of the 10.2.0.5 patch, a new package, DBMS_AUDIT_MGMT was included that allows you to move the
sys.aud$ table to a user Tablespace. If you are at this patch-level, move the audit table, plain-and-simple. Filling a user tablespace makes write operations against said user tablespace halt, but it won't bring everything else down like the SYSTEM tablespace filling will. This functionality was backported, but not to all 10.2 versions. You can check Doc ID 731908.1 on support.oracle.com to get all the details.
If you don't have the ability to upgrade to 10.2.0.5, you will need to regularly purge the
sys.aud$ table by some other means so that it doesn't continue to grow and consume space. If it gets too unruly and you find you do need to "shrink" the table, you will likely need to TRUNCATE it to release the white space back to the tablespace. I've tried shrinking this file using the typical trick of enabling row movement and shrinking the table, but because records are often appended to the end of the
sys.aud$ table and are added regularly, it often doesn't shrink much, if at all, which is why a TRUNCATE operation is often the most effective method here to free up some space.
Finally, your last option is to just disable the Audit. If it's not needed, this is probably the easiest option available to you.