Unless they told you to get off, this was a voluntary, not an IDB. UA pushes the DBC-free tickets on volunteers, but I never take them. Not only do they not accrue miles, but they are also capacity-controlled. You've learned an important lesson:
1. Don't volunteer unless you KNOW exactly when you'll be traveling and you're okay with that.
2. Unless a DBC-free is useful to you, insist on vouchers. If they won't give you vouchers, say, "No, thanks." For an overnight at my home airport, I ask for $600 in vouchers. If it's not my home airport, I ask for a hotel for the night (and I specify the hotel) and meal vouchers in addition to the $600.
3. If you are ever actually IDB'd insist on CASH -- not vouchers, not DBC-frees. That's the law.