Today was to be the day when I finally took the restoration project out of the garage up the driveway and back down. No such luck!

When the engine is not running I am able to clutch and shift into all four gears. When I start the car, I am unable to shift into any gear, 1,2,3,or Reverse. If if try, I just get the gears starting to grate on all four gears.
According to my father, this car has a multi-plate clutch? (what ever that is) and I should just keep on exercising my left leg and continually change gears when not running in the hope something will occur.
I should add that this would have been the first time in 8 plus years that the car has gone anywhere under its own power.
Is anyone able to clarify what may be happening and the novice mechanics way to fix same?
Thanks
John