On startup, getting ORA-00959: tablespace 'UNDOTS' does not exist

 Doing weekend maintenance on a Production systemI ran out of undo space while doing a large datapump import. I killed the datapump import job, switched undo files successfully (to a smaller one), and then when creating a new undo file, I killed* that* job accidentally. It rolled back partially, at least the datafiles that it had created were gone.  I thought I would shutdown and restart the database and then rebuild the larger undo tablespace again but then when I tried to shutdown and restart the database, it's telling me that the 'UNDOTS" tablespace does not exist. The datafiles are