MetaMask makes Dapp adoption quick and easy because we host a blockchain connection by default, and our trusty friends at Infura do an amazing job making this experience feel totally magical, most of the time!
Sometimes, some dapp usage has grown much faster than we've been able to anticipate, and so MetaMask's default provider connection can become intermittent. Sometimes we have to limit abusive sites.
At times like that, there are three options:
1. Wait and be patient.
2. Connect to another blockchain provider (like your own local node)
3. Use another client.
Connecting to another blockchain provider
To connect, you'll need to run a local RPC server, and then connect to it with MetaMask's provider menu (in the top left corner).
Parity and Geth both run on localhost:8545 by default, so this menu item can be handy: