Try it in the GUI's console first:

• Go to "Window->Console" and select "wallet: (none)" (not required)
• Then type: loadwallet "wallet_name" true

You can change "true" to "false" if you do not want to keep the wallet loaded in the next session.

In the command line, use bitcoin-cli with the same command as the above.
But in order for this to work, you must use --server=1 arg with your bitcoin-qt or server=1/ in 'bitcoin.conf' file.
In case you've set a custom data directory and not using rpc username and password, --datadir should also be indicated for it to find the ".cookie" file in that directory.