If you see these errors, you most likely have a corrupted redo log file:
ORA-01172: recovery of thread 1 stuck at block xx of file x
ORA-1151: use media recovery to recover block, restore backup if needed.
For ORA-01172, the cause may be that the block was corrupted and then repaired during recovery.
To attempt to fix this, do a RECOVER DATAFILE for the file containing the block. If this doesn't fix the problem then you may need to restore the file from a backup (and then attempt to recover it).
If the redo logs are still available, try doing a RECOVER DATAFILE x (where 'x' is the indicated file id in the error message).
If the database is completely frozen (unresponsive), try these steps:
1) SHUTDOWN IMMEDIATE
2) STARTUP MOUNT
3) RECOVER DATAFILE x
4) ALTER DATABASE OPEN
You may also as a last resort try RECOVER DATABASE.