They can put your post-dated check through your bank account and cause all your other checks to bounce. If your check to them bounces, they will eventually sue you for three times the face value of the check plus the costs of collection.
If you also have other debts that you are struggling with you may benefit from a consultation with a bankruptcy attorney to determine whether that is a reasonable solution for you.
The bottom, bottom line is a civil judgment against you. Nothing criminal.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
You can be sued for breach of contract and a judgment can enter against you -- which will allow PayDay Loans to garnish wages, bank accounts, etc.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Not being able to pay on a payday loan is a breach of contract. You don't need to worry about a "hot check" problem, but you will face harrassment on the collection of the debt. You might consider a bankruptcy, but you should speak to a lawyer about your situation. Good luck!
This general response is not intended to be legal advice because I don't have all the facts. The particular facts in each instance will change the recommendation significantly. Any statements made in your posting on Avvo are not protected by the attorney-client privilege because they are shared with third parties. I require a written contract for legal services, so an attorney-client relationship may not be presumed merely by my response to an Avvo posting.