By stating someone must of hacked your internet conncetion, you are then admitting something may have been uploaded or downloaded with your connection without copyright ownership by you.
As such you write back with a little humilatity and ask them for full details of what it was, when it was, and how exactly they believe you have potentially allowed this to occur!
Importantly after asking this also ask which company are they representing in this case in relation to the copywrite ownership and ask for proof of them acting for said party, and have in fact spoken to the copywrite owners for them to act on their behalf to go after you for any losses they feel they have lost.
Also then ask what basis do they evaulate the loss at that figure, as surely goign to the cinema, or renting a dvd, or buying the dvd or book or whatever it was would only cost a few euros/pounds/dollers etc.. and as such the coywrite owner in fact then only gets a small percentage of that figure and as such their losses would only be a few pennies, cents, etc... and not 000's of euros,pounds,dollars.
Upon them providing all these details you will then fully investigate their claims and try to identify for them whether it was done by mistakeby you, or member of your family, or someone who may have accessed your modem,router to share your internet connection, but assure them based on what minimal information they have provided so far you cannot accurately recall one way or the other on this issue until fuler information is provided.
They have to prove it was you and what their losses are, so ball is always in your court, upto near court action starts, but if that is the case, get rid of your pc, and storage devices and get new ones, this way they will have no proof it was you who did it at all!
As stated as an individual you are only downloading once therefore so not worth 000's if it was due to you uploading then ask them for proof of it and how many times also it had been downloaded by people and as such how they worked out the figure they claim to have lost, and if they are asking you to cover all those losses then you want it in writing all IP addresses who downloaded and the owners of those IP addresses so you can claim from them for illegally downloading something that you may have stored for your own usage! and ask for confirmation that they will not be pursueing those people who downloaded your upload, as the copywriter cannot have double bubble compensation by claiming from you and then from those who downloaded form you, if they are asking for more than a few pounds,euros,dollars etc...
Hope this helps