My front yard and back yard receptables have no power. What are some of the causes? They are GFI with no reset buttons.

How do you know they are gfi recepts if they do not have a reset / test button on them?

There is another GFI somewhere, and it is tripped.

That GFI could be in a bath or some other place inside.

Possibly a GFI circuit breaker also.
When I had a similar problem with o/s receptacles, after a lot of searching I found a tripped GFI outlet that controlled the o/s receptacles which had been concealed when the previous owner installed a suspended ceiling.