To resync to the revived Ropsten chain, at the command line run:
geth --testnet removedb
geth --testnet --fast --bootnodes "enode://20c9ad97c081d63397d7b685a412227a40e23c8bdc6688c6f37e97cfbc22d2b4d1db1510d8f61e6a8866ad7f0e17c02b14182d37ea7c3c8b9c2683aeb6b733a1@52.169.14.227:30303,enode://6ce05930c72abc632c58e2e4324f7c7ea478cec0ed4fa2528982cf34483094e9cbc9216e7aa349691242576d552a2a56aaeae426c5303ded677ce455ba1acd9d@13.84.180.240:30303"
or for Parity:
parity db kill --chain ropsten
parity --chain ropsten --bootnodes "enode://20c9ad97c081d63397d7b685a412227a40e23c8bdc6688c6f37e97cfbc22d2b4d1db1510d8f61e6a8866ad7f0e17c02b14182d37ea7c3c8b9c2683aeb6b733a1@52.169.14.227:30303,enode://6ce05930c72abc632c58e2e4324f7c7ea478cec0ed4fa2528982cf34483094e9cbc9216e7aa349691242576d552a2a56aaeae426c5303ded677ce455ba1acd9d@13.84.180.240:30303"
or for cpp-ethereum:
eth --ropsten --kill --pin --peerset "required:20c9ad97c081d63397d7b685a412227a40e23c8bdc6688c6f37e97cfbc22d2b4d1db1510d8f61e6a8866ad7f0e17c02b14182d37ea7c3c8b9c2683aeb6b733a1@52.169.14.227:30303 required:6ce05930c72abc632c58e2e4324f7c7ea478cec0ed4fa2528982cf34483094e9cbc9216e7aa349691242576d552a2a56aaeae426c5303ded677ce455ba1acd9d@13.84.180.240:30303"
If you have problems syncing to the right chain, you can force your client to connect strictly to peers known to be on the revived chain. This only needs to be done for the initial sync.
For geth, start the client without connecting to any peers:
geth --testnet --fast --nodiscover
Then in the console (geth attach), add them manually:
admin.addPeer('enode://6ce05930c72abc632c58e2e4324f7c7ea478cec0ed4fa2528982cf34483094e9cbc9216e7aa349691242576d552a2a56aaeae426c5303ded677ce455ba1acd9d@13.84.180.240:30303')
admin.addPeer('enode://20c9ad97c081d63397d7b685a412227a40e23c8bdc6688c6f37e97cfbc22d2b4d1db1510d8f61e6a8866ad7f0e17c02b14182d37ea7c3c8b9c2683aeb6b733a1@52.169.14.227:30303')
For parity:
echo -e "enode://20c9ad97c081d63397d7b685a412227a40e23c8bdc6688c6f37e97cfbc22d2b4d1db1510d8f61e6a8866ad7f0e17c02b14182d37ea7c3c8b9c2683aeb6b733a1@52.169.14.227:30303\nenode://6ce05930c72abc632c58e2e4324f7c7ea478cec0ed4fa2528982cf34483094e9cbc9216e7aa349691242576d552a2a56aaeae426c5303ded677ce455ba1acd9d@13.84.180.240:30303" >> ropstenpeers.txt
to add the peers (one per line) to a text file, and then run the client with flags:
parity --chain ropsten --reserved-peers ropstenpeers.txt --reserved-only
You can check that the client is on the revived chain by comparing to the block numbers on http://pool.ropsten.ethereum.org and http://ropsten.etherscan.io.