The ORA-3113 error is a generic error indicating that your session with the database has been severed. The most common cause of this error is a network error. You will need to be sure that your network connection to the database is not being severed for some reason.
The second most common cause of this error is an Oracle bug. You did not include an Oracle version, so I cannot verify if a bug exists and is causing your problem. Please check Metalink to see if a bug exists for your version. I would search on "ORA 3113 index rebuild" in this case. If a bug exists, Metalink will let you know which patchset or version contains the bug fix. You may have to apply a patchset or upgrade to resolve this problem.
If you cannot find a bug on Metalink, then your can file a TAR with Oracle Support. Alternatively, you may attempt to drop and recreate the index.