A storm can cause a power surge
that affects the electronics of a computer. This can result in data loss and possibly permanent damage to the computer. The most common method of preventing this is to use a surge protector or a UPS and, of course, regular backups to a USB and/or online drive. Newer versions of mIRC have been updated over the years to try to mitigate data loss where power is suddenly lost but there is only so much an application can do in this situation.