Motobecome Mirage (I think) made in France
There are fixed gear bicycles don't freewheel. Presumably you have multiple gears, so its not fixed gear bicycle (or you know it used to freewheel). 

That being said, if it is really a French Motobecane (the Motobecane brand is now used for bikes sold directly from some Chinese factory), it probably predates the common use of a freehub and almost surely uses a freewheel. 

The solution in this case is to put on a new freewheel. You can try blasting it with WD-40 or whatever to see if it loosens up, but at the end of the day you will need to replace it. Assuming this bike was sold in an export market (i.e. not in France), it almost surely uses a regular old freewheel (ISO threading, 1.375 x 24 TPI). If it was sold in France, it may require a French threaded freewheel (1.366" x 25.4 tpi; in which case, you're best off buying a new wheel and possibly spreading the frame to accommodate it). 

See these directions on how to remove a freewheel (and if you can't find the appropriate tool quickly or cheaply, use the destructive method).