Access all your existing My Maps by typing google.com/mymaps straight into the omnibox
...or...
Go to google.co.nz/maps or google.com/maps and go to the dropdown menu and find Your places and then Maps in the top navigation
...or...
Create a new map directly from your Google Drive. Use the New button dropdown, select More and then Google My Maps. Please note that this creates a new map - it won't take you to your existing maps