Your database crashed with the following errors after you issued the command:

> ALTER DATABASE DATAFILE RESIZE...

Datafile is ONLINE, and in a tablespace that is being used. In this
case the rollback segment tablespace.

ORA-00603: "ORACLE server session terminated by fatal error"
Cause: An ORACLE server session is in an unrecoverable state.
Action: Login to ORACLE again so a new server session will be
created

ORA-00376: "file %s cannot be read at this time"
Cause: attempting to read from a file that is not readable.
Most likely the file is offline.
Action: Check the state of the file. Bring it online

ORA-01110: datafile :
Cause: This message reports the filename involved with other messages.
Action: See the associated messages for a description of the problem.


Problem Explanation:
====================

When using command ALTER DATABASE DATAFILE RESIZE..., your instance should
have the database mounted, open, or closed, and the files involved should
NOT be in use.

With rollback segments being used in this ONLINE datafile, attempts to resize
it could cause a crash and even corruption.