Check that all the battery connections are clean and tight before you do anything, particularly check the grounds and especially the one to the engine. Do all of this before you start using jumpers or anything else. Then top up the batteries with distilled water.
Try it now. If no joy, test the solenoid by jumping the large terminals - careful, if it works, the engine will start and so be prepared for it! Watch out for things that spin and wear ear protection! If the solenoid is bad, they are cheap and easy to change, turn off or disconnect the batteries first. I always disconnect the electrical connection to the fuel shutoff too, so it can't start by mistake. You could do that for your solenoid check too, if you wish.