The answer is the same as many tax questions, "it depends".  If this was a personal loan and had no business connection then you possible have a personal loss, which is not normally deductible.

If you lost money from investment or a loan for a business then you may have a deductible loss.  In order to take a business bad debt (ie. you are in business and made a loan to someone or did not get paid) you must have included the money in your income in a previous year.  Most business people fail this test because they are "cash basis" taxpayers.  This means you include money in income when you have it in your possession.

If you made a loan to a business or a person for a business reason you may have a deductible debt that you can take as a capital loss on Sch D, however, there are very strict documentation rules.  You must be able to show the following:

1.  You gave the money as a loan, not a gift.  There should be a loan agreement in writing with a realistic interest rate.

2.  You have exhausted all methods of recovering the money.  You've written certified letters, taken the person to court for small claims, been listed as a creditor on a bankruptcy, etc.

3.  You must realistically be sure the debt will not be repaid.  If you take the deduction and the money is then partially or fully recovered you must take this back into income in that year.

Talk to your tax advisor about the details of your specific situation.  You can locate a great one using the search engine at the top of the page.