First you have to understand that many people depend on computer systems to get their work done. The computer systems they depend on are known as production systems. For example the local banks need to have computers process their checks so that people can get paid and so that the right amounts can be taken out of the employer bank accounts and deposited in the employee bank accounts. The poor bank employees don't know whether the intruders have damaged some of their bank records or programs. The only thing to do is to check.
Security experts say that such a system is no longer trusted.
The employees have to reestablish trust in the data and also in the programs. Criminal hackers have been known to insert their own changes to certain computer programs. Criminal hackers often leave what are called back doors into the systems they've already broken into. Back doors allow the hackers to re-enter the compromised computer systems anytime they want. This kind of change to system software is a real threat to the people that have been victimized.