Bitcoind could not locate RPC credentials but rpcuser and password are set
I found I had to copy the contents of ~/.bitcoin/bitcoin.conf into /etc/bitcoin/bitcoin.conf because that's what's being used when bitcoind starts on boot up. I put my user and password in that file, rebooted, and bitcoin-cli worked again.

Also try adding

bitcoin-cli -stdinrpcpass getblockchaininfo -rpcuser user -rpcpassword password