Refund your Oyster or help charity
Well then you should check out if you can get the spare money back, as there's £124 million on them (not yours, but loads in general)
Transport for London reckon - out of the 75 million cards issued since time began (2003) - that there's around 28 million cards that haven't been used for over a year, and THESE have a combined amount of £64 million on them. Lordy. TfL also has £59.5 million in deposits on these cards too.
Once you've found your card, here's what you can do:
Phone: call 0343 222 1234 with your Oyster number to hand and go through all the security questions, get the nice person on the other end to cancel your card, which you then send to TfL's customer services HQ to get a full refund of your credit and deposit via bank transfer, cheque, or TfL website credit.
Post: Complete a TfL refund form and send it in to TfL with your Oyster. (Admittedly, if you live in, say, outside of London, a TfL refund form may be difficult to come by).
Person: You can apply for a refund at any London Underground ticket office (BE QUICK WHILE THEY STILL USE HUMANS) and someone will be able to help you.
If the payment method you originally used was a debit or credit card that's since expired, you can ask for the refund to be transferred to a nominated bank account or to receive it via cheque. If you haven't registered your card, you'll need to show some form of ID in order to get the refund. You can't get a refund on behalf of someone else.
If you can't be arsed to do any of this, and fancy being nice, TfL donate unused cards to charity, and the spare cash goes to the Railway Children charity which helps nippers on the streets of Africa, India and the UK. Which is quite nice isn't it? (Although you'll probably go for the refund though eh?).