Whenever you start an indexing process, Magento writes out a lock file to the “var/locks” folder.
The lock file prevents another user from starting an indexing process. However, if the indexing request times out or fails before it can complete, the lock file will be left in a lock state. To short out this issue check the last modified dates on the lock files to make sure someone else is not running the re-indexer right now, and then remove the lock files. This will clear up your error. After that, run the indexers one at a time to make sure each one completes.
You can also find other re-indexing issue ans solution on Magento Re-indexing problem.