Yes your best bet is going back to Bangkok. From there you have a number of options, bus, train and plane. If you take a look at the FAQ at the top right of the page How to get to/from Koh Tao it gives a comprehensive list of options.
Here's the link in case you're accessing the site from a smart phone
Safe and happy travels
Yes, as Lynds suggests, plenty of options. Either the night train or bus from BKK to Chumphon and then the morning Lomprayah service to Koh Tao is normally what we recommend to our customers. The flashpacker alternative is to fly to Samui with Bangkok Airways and ferry transfer to Koh Tao from there.
Not sure when you are travelling, but as we are coming into peak season now, I'd recommend booking in advance to avoid any upsets to your schedule.
Safe travels :)
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