I think that's a shitty outlook. Of course your startup has odds of failing, but if you quit on your idea because of this hiccup, your chance of failure is 100%. These things happen. Be 100% up front with your investor, tell him you need some time to find a new co-founder, and then go find one. If the investor can't stomach this challenge and wants his money back, all good - his money wasn't worth the hassle of having an investor with a low risk tolerance. But perhaps he's someone who either believes in you or your idea, and would want you to persevere and get this thing off the ground, not jump ship at the first sign of difficulty.
Your other options are to hire some freelancers/developers to help you take your early stage prototype to a public-facing finish line. Or learn to code yourself, that's always an option ;)