What to do
First confirm the cancellation in writing and ask the airline what alternatives they can give you on their own network. At the same time open a map and check train and long-distance bus options between the same two cities or nearby hubs. Decide quickly whether you need to arrive the same day or can stretch into the next morning. Keep all receipts if you pay for anything yourself, because some costs can be claimed later.

Three options
First option is the next available flight with the same airline or a partner. This is often the easiest to arrange through the airline app or counter. Travel time stays close to the original, but you may face a long wait at the airport or an overnight stay. Rough extra cost is usually the change fee or nothing if they rebook you for free, plus possible food and hotel if the delay is long.

Second option is a direct train if the route is served by rail. Many city pairs in Europe and some other regions have decent train connections that take between four and eight hours. You avoid airport queues and can work or rest on the way. Ticket prices for a same-day or next-morning seat often sit between 40 and 120 euros or dollars depending on distance and how early you book.

Third option is a long-distance bus or a mix of bus and local train. This is usually the cheapest and most flexible when trains are full or the rail link is weak. Journey time can stretch to six or ten hours, but overnight buses exist on many corridors. Expect to pay 25 to 80 euros or dollars for a seat bought at short notice.

Cost
The airline rebooking is often free when they cancel, but anything you buy yourself (train, bus, extra night) comes out of your pocket first. Keep every ticket and receipt. Some of those expenses can be claimed back later under passenger rights rules if the cancellation was within the airline’s control. Factor in possible hotel or meal costs if you end up waiting overnight.

Tips to cut losses
Act in the first hour after the cancellation notice while seats on trains and buses are still available. Compare total door-to-door time, not just the main leg. If the airline offers a hotel, take it only if it actually saves you money and time. Screenshot every offer and every refusal. If you buy a new ticket yourself, note the reason clearly so the refund request later is easier.

Summary
A cancellation twelve hours out is stressful but rarely leaves you with zero choices. Check the airline’s next flight first, then look at trains and long-distance buses in parallel. Pick the option that gets you there with the least damage to the rest of the trip, keep every receipt, and move fast while seats still exist. Most people who run these three checks end up with a workable plan before the original departure time even passes.