Simple to diagnose:

1. Go to the GFCI. Unplug anything that may be plugged into it. Measure the voltage on the line screws. Some won't reset if there's no incoming power. If there's no incoming power...check the breaker and then wiring leading up to it.

2. Remove the wires from the load side. If it still won't reset, GFCI is bad, replace it. If it resets, then you likely have a ground fault on the load side.

3. Make sure nothing is plugged into any of the downstream recepatcle and reconnect the wires to the load side. Now you are going to have to go from receptacle to receptacle in sequence. Make sure that no grounds and neutrals are touching in the GFCI box or any of the others.

4. If visual inspection yields nothing, disconnect the wires feeding the first receptacle after the GFCI. Try reseting it. If it still trips/fails to reset, the wires between the GFCI and the device have a fault. Look around for possible damage such as screws/nails driven into the walls, etc...

5. If it resets, reconnect the receptacle to the supply wires (with the others removed). If it trips, bad receptacle. If not, connect the wires feeding the next receptacle and repeat 4/5 on the next.

Note that the garage GFCI may serve the basement and the outside receptacles. In some older construction, it may serve the bathrooms.

The breaker is on, and didn't trip

Are you absolutely sure? Sometimes breakers don't give a visual indication that it's tripped. Did you flip it fully off and then back on again?