If you are able to click on the cogwheel and then "Abandon Trip" it should work to then start a new one.

If not then the following should resolve it:

Open up Developer Tools in Chrome or Firefox and go into "Console" and write the following:

localStorage.clear();

And press enter. Then try again to open up Singleplayer.

Here is how to open Developer Tools and Console - https://balsamiq.com/support/faqs/browserconsole/