My main quest is gone, and I don’t know how to get back to it! I only have krokotopia unlocked so far, if that helps.
You could ask support but that will take time waiting for an answer so try this first...
Click here to bring up the Krokotopia Quest Tree. Every quest on there is a hyperlink. You can click on it and it will take you to a page that lets you know everything about the quest, including who gives it and who you hand it in to. You're likely to have lost your quest line either at the point of handing in or (more likely) at the point of picking up.
The NPCs you pick up from or drop off your quest to are also hyperlinks so you can easily find exactly where they are located.
Look on the quest tree for the last area you remember being in and click on the quest finder in your quest log. Check on the quest tree if it is a main quest, if it is you're sorted, if not you can ignore it and start going to the NPCs who give/take in quests in that area. Even if they don't have the icon above their head click X when you're near them to check if they can speak to you. Hopefully you will find your main quest quickly and easily this way
If you don't mind picking up side quests you could use quest finder to pick up any and all quests in the world that you have not yet completed.
If you may have completed Krok you might want to check Sergeant Major Talbot in the Oasis to see if he gives you 'Missing Souls', if your world gate shows Marleybone as an option go there and speak to Private Kinchley in Wolfminster Abbey.