Can you be jailed for an unpaid loan or other debts?

So I took out a payday loan because money has been really tight and I have been trying to get money matters fixed. I have had the option of requesting more time every 15 days so far and had to pay anywhere from 40-60 dollars to keep the loan ppl happy. Well my account went negative before they tried to take the... show more So I took out a payday loan because money has been really tight and I have been trying to get money matters fixed. I have had the option of requesting more time every 15 days so far and had to pay anywhere from 40-60 dollars to keep the loan ppl happy. Well my account went negative before they tried to take the most recent payment and when they tried to debit it from my account it came back and they emailed me saying it bounced basically and I can't pay them just yet. It will more than likely take a few weeks before I can really do anything. Can they have me put in jail for this? They tried calling me too and I googled it and found other people showing letters they received saying that they would have the FBI investigate them and try to prosecute to the fullest extent of the law... etc etc.
That kinda freaks me out. Does anyone know what they can or cannot do? I am going to try to get on top of everything but need a few more weeks before I can do much.

What should I do?
Update: Also... Because at least one person pointed it out I did NOT write them a check. They make you give them your bank account information nowadays and they just take it out like you would get a direct deposit... Except instead of a deposit it's a direct debit by them. And I didn't have money in there for them... show more Also... Because at least one person pointed it out I did NOT write them a check. They make you give them your bank account information nowadays and they just take it out like you would get a direct deposit... Except instead of a deposit it's a direct debit by them. And I didn't have money in there for them to take but there is no stopping them from trying unless you can pay them the extension fee which I don't have right now so my bank did a returned funds thing and now I'm negative a bunch. And that is why they are starting to call and email me. Because they didn't get their payment.
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