It's bad enough that you worked hard all week and may have had to deal with annoying co-workers -- now you're having problems getting paid. Well that's simply unacceptable — the law requires employers to pay employees for work completed. If you find yourself in this situation, take action immediately so that you can get paid.
Contact a representative at your employer's payroll office first to ask why your pay is delayed. If this is an hourly job, confirm that payroll has the correct information about time you worked; if not, provide proof of your work hours (such as a punched time card or logins to work software) and a statement in writing. If it's a salaried job, find out whether there is an overall issue with everyone getting paid or just your specific employee account. Work with the representative to get the problem resolved.
Contact your supervisor and the human resources office in person and in writing if your efforts to fix the problem with the payroll office directly still leave you unpaid as of the next pay period. Ask the HR office to work on your behalf to resolve the issue if they agree that you're entitled to payment for work you've performed.
Call your union representative (if applicable) to resolve the issue with unpaid wages if working with the HR office does not help. The union is commonly charged with handling escalated situations like this with the employer and acts as your representative to resolve the issue. The union representative may schedule a sit-down meeting with you and the employer to discuss your nonpayment issue.
File a claim with your state's department of labor, labor board or the similarly named unit in your area. Fill out any forms required and explain your situation. You may have to provide detailed proof that you're entitled to the payment from your employer so that the labor department can investigate the matter.
File a small claims case in court if your efforts aren't fruitful to collect the payments you're due as another option if your state's department of labor doesn't handle these matters. Note "unpaid wages" as the reason why you're filing the case. Keep in mind that this can be a complicated matter if you still work for the employer so you may want to consult with an employment lawyer before taking this step. The Fair Labor Standards Act prohibits an employer from retaliating if you file a claim for unpaid wages.