1. you must have set 
   log_archive_start=true in initsid.ora
2. so u have enable archiving.
3. But u must not have specified a valid 
   log_archive_dest ( a destination for
   the archvied logs )
   or 
   the desitnation is full
   or 
   something is wrong with this.
4. so oracle does continue to function normally,
   unitll the contnents of the current log is archived.
5. either turn of archiving or
   look into clearing the archiving.              
6. you can force the logswitch
   using
   alter system switch logfile;