Your computer might be safer disconnected from the internet, but never absolutely safe. If you care about your data, back it up to an external disk, or flash drive from time to time, and put that in a drawer. Or better, a drawer at work, or a safe deposit box. The backup gives you insurance against the computer breaking down from normal failures, like the disk going bad. Keeping the data in a remote location makes it less likely that you’ll lose the data if (God forbid) your house burns down.
In theory, if your computer was once connected to the internet, it could have picked up some kind of time bomb malware, say something set to corrupt the disk at a significant date in the future. Or if you bring data onto the computer in any other way, a virus could get in that way. Remember, there were viruses that once spread by floppy disk, before people generally had internet.