To determine if you have a bad debt or a capital loss, you must first determine if the amount was actually a loan or whether it should have been classified as a capital contribution.
If it was a loan it will probably be classified as a business bad debt. See link to IRS Publication 535 below. If a non-business bad debt, see Publication 550 (link below).
If the amount should have been a capital contribution, you will have a capital loss on disposal of stock.
Business Bad Debt, see page 37: http://www.irs.gov/pub/irs-pdf/p535.pdf
Non-business Bad Debt see page 53: http://www.irs.gov/pub/irs-pdf/p550.pdf