Firstly, they have to give grounds for the breach. What do they think you've done?
Parked outside marked bay? Overstayed? Not bought a ticket?
If it's not bought a ticket, those type of machines are notorious for these problems.Between the person making an error, and not putting right reg number in, and some ways machine can get it wrong.
So if they claim you haven't paid at all, and you're sure you did, contact them, tell them there's been an error, and can they check the records, to see if a reg close to yours but a bit wrong ,did in fact buy a ticket. Ask them, politely but firmly, to correlate their ticket machine data, and their number plate cameras data. Help them by giving as many details ,time of entry, time of leaving ( ranges if you don't know exactly), reg , type, colour of car.
This may do the trick.
Read the notice. If it specifies how, when and to whom you must appeal, make sure you do so.