Brudgers is spot on, and #5 is the most likely outcome. If their rich guy investor has cold feet, it's probably because they're running a shitshow not a company. Or rich guy investor actually doesn't have money.
That said, my additional random internet two cents for the inadvertent consultant turned banker for his/her client:
Do not do another thing for them until they pay. You can make it nice, couch it in terms of "my employees and I can't eat, fuel our cars and feed our children with promises. Let us know when the money comes through." And say it in a much nicer way than I just wrote.
If you have any materials including source code commits, deliver them on request. Holding things hostage gives them a leg to stand on should you need to go to court, or could be cause for them to sue you. "We withheld payment because they didn't deliver the code". You've got nothing to lose by delivering their materials. Get receipts or written acknowledgements of anything you return, with a time and date on them. Send your own acknowledgement email of the receipt so they can't claim the receipt is forged.
Then you might consider filing a case which they probably won't fight. You can maybe just sue for $5000 in small claims, I know $.20 on the dollar sucks but it's sucks less than $0. But you can file and obtain a lien for less than $200 in small claims. You're probably looking at dropping $7500-$10000 in retainer for an attorney to sue for entire bad debt, but your resulting lien would be worth $25K + fees. Once you have the lien, you can look at selling it to a debt collection agency for probably $.30 on the dollar. They might actually still have some money and will pay you to get rid of the lien, as this will certainly scare off any investors during due diligence.
And here's the non-two cents, sage advice:
You've been a consultant for 10 years. It's time to start acting like a grown up business man or woman.
Always work from a contract. If the client says bullshit like "We want to keep this friendly", remember that contracts preserve relationships. Walk away from clients that want to work on a handshake, any reasonable businessman or woman does the same and understands why you want a contract, and won't begrudge it. The begrudgers are amateurs at best and con artists at worst. Doesn't matter because either way, they will drag you down with them.
The contract probably won't be worth enforcing, but it will proactively create the understanding about why you walked away and make the exit easier for you to deal with. And it will likely keep you from getting sued or protect you in case some loser gets the idea that suing you is their payday. And just remember, IT'S BUSINESS. It's not personal. You can still maintain a friendly relationship or even a friendship with clients while working under contract. My experience is I commanded more respect after requiring contracts, even with existing clients. It's like I leveled up 3 or 4 levels, just by hammering out a two-pager and making them sign it.
Once you have the contract, enforce it. I usually structure mine into thirds, one third up front, one third at the halfway point, and one third upon completion. Works really well for fixed-rate contracts. If you're billing hours instead of fixed rate, work on a retainer. And stop work per the contract if they don't refill the retainer in a timely fashion. Again, time to grow up. You don't really want to be Bank of the Unfortunately Indigent, Inc. do you?
IF you have a particular affection for a project or client, THEN you can grant some forbearance in terms of they pay half of the weekly billing every week or you're out. This is more gambler than banker, lol. You decide up front, you're willing to walk away without getting paid the rest. Cash flow problems are a reality many businesses face, and being a resource in a time of need is a bonding process and can create priceless goodwill. Gamble wisely...