I would get a representation submitted on TFL's website before close of business today, Thursday the 11th. Today being the expiry of the discount window that's stated on the PCN.
If you challenge within the discount window then TFL normally re-offer the discount if they reject. So you might as well have a go.
The Blackwall Tunnel has been free of charge for maybe a 100 years. The charge was only introduced when the Silvertown Tunnel was opened on the 7th April. We've had a number of cases on here where PCNs have been cancelled for a first offence.
So tell them you're terribly sorry, however you did try and pay but the system errored out. Ask them to use their discretion and cancel the PCN. Enclose a copy of your error message. See what comes back.