Why bike will not move in any gear? - 1980 Suzuki GS 750 L

The derailleur mechanism, as its name implies, 'derails' the chain from one rear sprocket to the next, either larger (for a lower gear) or smaller (for a higher gear). It is spelt that way as it is a French invention.

The clutch is not engaging by the sounds of it ,does the clutch cable have some free play at the lever ?

when riding? if so then you maybe moving to fast for this gear to down shift and or you may have a bad spot on the gear

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sounds like you need to rev your bike a bit more , don't be afraid to stretch its legs in the lower gears this help keep plugs and cylinders clean.

The problem has nothing to do with the derailleur itself. Either your shift cable has stretched and is now too loose (in which case the shifter will move but have no effect on derailleur movement), the cable/housing is rusted (in which case the lever will usually not move at all), or the lever is broken or gummed up (in which case it will move, but you will get no clicks). Each problem has a separate solution. Either take to a bike co-op for help in fixing it, to a bike shop to have them estimate and repair it, or go to a site like Park Tool's Repair Help and learn how to fix it. It's very difficult to instruct you with just words, and good options are already available.

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Your bike must not be the Cruiser pictured - that's a one-speed bike. So here's a general description--- For gear changes on multi-speed bikes, where the chain has to move between different size sprockets, the chain must be moving in order to shift. So, yes, you have to be pedaling, but it works best if you pedal very lightly - so that the chain is moving, but is under very light pressure. On the other hand, if you have a 3-speed bike where the gear changes take place inside the hub of the rear wheel, the wheel should be moving, but the chain should not - so you should be coasting to change gears.

You have to look if when you act on the shifting level it get hard to move or it is free, if free, probably the cable is broken.
Also check if the cable is not too loose, try to follow the cable track and move the level and see if the cable move with that, in that way probably can find where the cable is stuck or broken/loose.

If the cable is rusty or otherwise binding it won't let the derailleur fall toward the smaller sprockets when the control is moved. Likewise, it may not even let you move the control in the direction that pulls the cable (bigger rings).

If you don't know much about the process I suggest you read this:

http://coachlevi.com/cycling/complete-beginner-guide-to-bicycle-gears-shifting/

and take the bike to a Local Bike Shop for a look-see and maybe a tune-up.