As far as I know at this time , no-one really know what caused the system to crash , the root cause I mean , not yet but likely by tomorrow or in the next coming days we'll know. From my experience in the last 40 years or so the Flight plan processing systems (FDPs) are generally crashing following a system update. one line of programming is wrong ,and when an external factor comes in . it causes the issue. This could happen at any time , generally when the system is peaking . Typically system updates are done at night, tested for a few hours , then if OK put on line in the morning ,At least that how we do it in most centers. Done it for years in my own center. If it crashed later in the day we just reverted to the previous level which is on stand by on the back up computers, whole thing takes no more that minutes of an hour max to be back to normal. When if takes half a day or more then something is wrong on your processes or your system architecture. Could also simply be the result of cost cutting measures, like not replacing back up computers, or outsourcings maintenance and code writing ,to far away countries with cheaper labor , etc.. I am not saying that this was the case here in NATS, but I have seen this happening in other places recently .
Finally FDP system failures are not a unique UK/NATS issue, Geneva had a major failure some time ago, , Brussels, a couple of years back etc. even Roma had one also yesterday.at the same time as London , so we feared a wider cyber attack. But so fat it looks like the 2 were not connected. But if the investigation later shows they were, then we really are in the sh*t .