Platform: Oracle on Windows 2000

We have a production system that has been running ok for a year until last night, the CPU usage for Oracle.exe shot up close to 100% AND stayed there for at least 10 hours. I suppose the fact it stayed high for more than 10 hours rules out SMON was doing cleanups and coalesing. The temp tablespace is about 2GB. There is no code change since day one.

Does anybody know any other situations that may have caused something like this?