According to the announcement of Ethereum Classic Labs, the Ethereum Classic network underwent a chain reorganization last night as a result of an attack on the network. And the issue has been resolved.
OpenEthereum/Parity and Open-ETC no longer function properly due to an unidentified bug, said the announcement. The bug caused a temporary and minority chain split. In the future, any Ethereum Classic node operating OpenEthereum or Open-ETC could result in mining the incorrect chain. The miner 0x75d1e5477f1fdaad6e0e3d433ab69b08c482f14e mined ~3500 blocks offline or ~12 hours. When the miner came online it caused a large chain reorg starting at block 10904147–10907761. OpenEthereum/Parity nodes were unable to handle the large chain reorganization causing them to go out-of-sync from the network.