Put the barrels and forearm back on. Open the barrels and they should recock the hammers. If the hammers don't reset, pull the barrels back off and inspect the cocking slide on the bottom of the lump to see if there is any damage to it. It should freely slide back toward the breech end and spring return to the original position. This slide is what engages the levers in the reciever slot and cocks the hammers. There is also a flat rectangle metal extension on the bottom of your forearm metal. It must engage with and push against the cocking slide on the barrel lump. You may have just re-installed the forearm without this extension pushing the cocking slide.
