Call shopify and wait on hold. They will extend your trial as a one time courtesy. But you will have to start paying in 7 days.
If if became unavailable before the trial was over, then they probably saw it was either inactive or something set off a spam alert or something along those lines.
If you are planning to launch your store later than 7 days from now, you should have a Shopify developer (like me) roll over the account for you, that way it can stay FREE as long as you are building the store, and you will only have to pay as soon as you open it to the public for sales, you can even have the domain linked up and people can see your store, they just won't be able to checkout until you launch and connect a deposit account.
Email me at email@example.com if you want to know more about that route.