The problem is I had (for mistake) clicked on the "stow" button of a grenade and it has disappeared.. that happened in the ship has been assigned to be piloted by me.. the fact is that in the room (the entrance room) where the grenade was there are no items where the granade could be put it (there are some other grenades) and I don't own no backpack or belt..
What can I do to get back the grenade?
You sure you didn't put it into your vest or something? Items can be stowed in many different things. If you didn't click anything after 'Stow' to select where to store it, it should still be right where you left it.