I have a monero-gui wallet: it shows my XMR balance, which is not zero. When I open monero-wallet.cli it shows my balance is 0.000. What is going on here?
This look like you have a balance on subaddresses but not on main address.
While the GUI aggregates all accounts addresses in a single balance, the CLI does not.
You could do a
show_balance all on the CLI (IIRC) and check if the balance is on one or many subaddresses.