Fabiana - there is one trick someone told me and which does seem to work (sometimes).

Select the dynamic cloth and press delete. Then do an Undo. This might bring back the cloth in simulated position.

I have had this problem once a while since Poser 8 (mostly with older scenes). Newer scenes are usually ok. 

If I have lots of simulations in one scene, I also do the same thing as moriador - spawning a morph - that way I don't have the recalculate if the sim gets destroyed (accidentily or otherwise)