If I loaned a business money, but they've declared bankruptcy (without making any payments), can I write off only the original amount of the loan or can I also write off the interest accumulated up to the point of bankruptcy?
If you personally made the loan, then you may deduct the amount of your loan that was not repaid as a loss. You need to be able to show that the loan will never be paid back and the collection efforts you made. Bankruptcy is a good indication that the debt will not be repaid to you. The IRS says "Generally, to deduct a bad debt, you must have previously included the amount in your income or loaned out your cash." Therefore, the interest accumulated, which you did not receive, is not part of your loss.
Click the IRS link below for more information on nonbusiness bad debts.