Simplest thing to do if you think it will work and don't need money is just sit on your vested shares, perhaps ask for a little more as an exit package, be supportive, offer to join the board as an advisor, and move on.
If you'd like a clean break and some cash, you could call your last round investors and offer them a discount to the last price for your vested shares. Depending on the state of the company and what's next, they may be interested -- there's an implication in these conversations that you'll find SOMEONE to buy eventually, which they may or may not like. I'd guess unless things are gangbusters, you might look at 30-50% discount from the last valuation, knowing nothing about the common, the company or the pref stack.
Another possibility - you could offer to sell your shares at a discount to your cofounder, leaving him in a good spot - he could ask around for a loan / investment (in which case the company would be the buyer, and he'd benefit pro-rata).
Personal advice - keep some, sell some, stay available to help.