First, I will make clear that I am not in your shoes and know every detail of everything, your market, clients, etc.
I do have a couple two cents to put in from your experience in the last 5 years.
One: Payment Terms. $18,000.00 in immediate materials and labor burden with a 45-60 payment to reimburse those expenses plus provide you with your profit. That right there to me is a huge red flag. I learned this lesson from charging customers 'per mowing'. Ex. Month of May mow account weekly, June 01 send invoice for services rendered from May 01-May 31, payment due June 30. At the beginning of each year there was always a struggle for money, a large overhead in labor, costs, mulch materials and having your accounts receivable on those terms. I went bold last year and said I am only offering maintenance contracts (contract is only way to set a solid price to invoice customers MONTHLY DEPOSITS), I did my calculations and knew I would loose 10-15% of my customers just because it 'seems complicated', but in return I knew I would be gaining more business from my customers by adding additional services. As far as projects go, I have payment terms (which are somewhat negotiated with the client) but 50% down 2 weeks before the start of the project to begin ordering/paying for materials, at 50% project completion a 25% payment is due; then immediately at the completion and 'walk-through' the final 25% is due. I have found this is completely fair, it is not forcing clients to pay for project before it is completed and they understand it is not making me have to buy the materials.
What happens when you do a $30,000.00 seal job and that customers check bounces? They may have had the 30k in the bank when they signed the contract, but 2 months later anything could have happened. I would never put my or my company at risk like that, just because they signed the paper to pay you whats due, does not mean that will happen 100% of the time, putting my companies profit out there like that to pay for materials with good-contract-faith to get paid in a near 60 days..YES, the bank screwed you over, but if you had good contractual payment terms, you would have had your money to pay your suppliers and not be relying on a line of credit for the mean time waiting on money to come in.
Two: Where's your profit? You said after materials, labor, and overhead was paid for you were left with 12k profit-job/week...and only $20,000.00 in the bank? Like I said, don't know the in's and out's of every detail, but sounds as though you grew to operating off line of credit and were profit spending savy, when you should have been working to get out of relying off the line of credit to keep the engine of your business going.
Hope most of everything has recovered well, look forward to hearing more of the story.