It's certainly worth calling the hotel. I had a reservation at a Hilton in San Francisco the night of a work party last spring, 2 day cancellation IIRC, and when my daughter got sick we couldn't go -- I expected to be charged anyway, but called at 2pm that day and explained, and the clerk didn't hesitate to cancel it for me with no penalty. I'm sure it depends on a lot of factors (I've never done that before), and a prepaid reservation may be different (since they have to actually refund the charge, instead of just eliminating the booking), but it can't hurt to ask nicely.