Check engine light (EVAP system)

A few weeks ago, my check engine light came on, and I figured out which code it was, and it was the code that says there is something wrong
with the EVAP system. The dealer said it could be a loose gas cap, or a leak somewhere. The light went off after a week or so, so I thought it was fixed. (See ).
Well this morning the light came back on. I haven't gotten any gas since this problem originally began, so it's not like my gas cap loosened itself. Does this mean I have an expensive problem I need fixed?
Thanks in advance for your help.
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