At first blush it seems there is no hint in the film. But the truth may be hiding in plain sight.
Personally, I, like others here, always assumed he was in deep trouble, in the form of a debt owed to dangerous people. Gambling, criminal enterprise, something surely has put his life at risk, leaving him desperate. But no such explanation is ever provided in the film, beyond his mention of being in "some trouble" at the beginning.
However, in an interview, the actor William H. Macey, who plays Jerry, suggests a simpler and much more wholesome explanation. Jerry wanted to build the parking lot (Macey refers to it as a parking structure, implying multi level garage), the suggestion is Mr Lundegaard's motivation is purely about gaining independence and success outside of his father in laws business and in his own right.
This tracks well in the film, the relief he shows when his father in law reports it looks like a good deal, his immediate interest in cancelling the kidnapping, and his deflation when it turns out his father in law wants to cut him out of the deal, leaving him with only a finders fee. Perhaps Jerry has already committed to this real-estate deal, signing an offer and submitting a (non refundable) deposit, explaining the fraudulent loans with GMAC.
In this vision we see an average, reasonably good man, betrayed by his need for capital. He is desperate not because his life or limb is on the line, but only his self image and chances of "success" hang in the balance.
He needed the money to buy that parking lot and become his own man, an equal with his father in law, and a success in the eyes of society. He was so desperate to establish himself as a "serious man", he risked absolutely everything, and lost.
Quote from the interview:
What made you want to play Jerry?
One reading of the thing and I totally understood Jerry Lundegaard. I thought: “I can endorse that.” He wanted to take care of his family. His wife is mouthy, and his father-in-law is overbearing and a prick. Jerry came up with this magnificent idea for a parking structure that was going to make somebody a lot of money, and his father-in-law, yet again, is just going to steal it from him. I think it was a good plan. It went wrong, but it didn’t need to go wrong. If it had worked and no one had known, all would have been well