Cancel a Hotel Room After Checking in Online

To answer generally, there is the official policy, and then there is what the hotel chain or individual hotels do in practice. If it is before the cancellation deadline, then officially you are permitted to cancel. If it is past the cancellation deadline, then you need to contact the hotel chain guest assistance desk or the individual hotel directly, and plead your case. If you have a pre-paid, non-refundable rate, it's more difficult, but sometimes an exception is made. If you need to cancel because your flight is delayed or a similar circumstance out of your control, you may be permitted to do so. If you ask to have your reservation shifted to be a day later instead of cancelled, that's often easier for a hotel to do. In my own experience, I've always been permitted to shift or cancel when a flight delay or bad weather caused the issue.