If your espresso machine is stored for long periods of time it's vibratory pump can dry-socket and fail to pump water upon reactivation. You will hear it running but no water will drain from the water tank or come out of the group head when attempting to pull a shot.
Note, if the vibratory pump is barely making any noise(a quiet hum) instead of the usual vibration sound the pump may have failed at which point you will need to replace it. Also, if the pressure gauge on your machine goes up to 9-10 bar pressure while attempting to run the pump then the pump is functioning but the machine may have a clog somewhere else.
To re-prime the pump first try turning the pump on and off several times allowing a few seconds in between. If this does not work you may need to inspect the pump manually. Observe the following instructions:
Fitting on vibratory pump should be removed...
Using a pick, remove white inner screen by turning it counter-clockwise...
Remove spring, gasket should be attached but may need to be lightly disturbed if mineral has stuck it in place...
Clear away any mineral buildup from small black mushroom shaped gasket. Re-attach spring to gasket, place it inside pump outlet, place white plastic screen inside and push it down until it seats fully in the pump (no need to twist it).
You can now put the pump back in your machine. Test before putting panels back on machine, if this process doesn't work you may need to replace the vibratory pump.