For instructions on connecting to your own local Ethereum node with MyCrypto, please see this article.
Verify the URL you entered is correct.
Verify the port you entered is correct.
Verify that if you are connecting to an http:// node, you are running MyCrypto locally (NOT over https://).
Verify you are running with:
geth --rpc --rpccorsdomain "*"
parity --jsonrpc-cors "*"
You can also run both clients in light client mode, which only downloads the recent state of the blockchain and saves bandwidth:
geth --rpc --rpccorsdomain "*" --syncmode "light"
parity --jsonrpc-cors "*" --light
Verify your node is actually up and running.
Try removing the node and adding it again.
Try changing the node in the MyCrypto interface to Etherscan or Infura, then change back.
Try using Google Chrome or a different browser.
Ensure that you do not have a firewall or something else that may be blocking the connection.