Okay, thanks for the details. Having had Windows installed to run Windows PC programs would not be an issue.
However, I do believe the age is a contributing factor; What you are experiencing, sadly, is not a software issue but rather a hardware problem.
Your iMac has developed a defect in the internal hard drive itself which is why you do not see it appear in the sidebar of Disk Utility.
Unless you are comfortable opening your iMac then this would need service. When troubleshooting hardware issues, sometimes the final diagnosis is that it is a physical defect in nature and the ‘answer’ such as in this case may be a recommendation to seek out repair from an authorized service center if the defect in question is not considered “user repairable”. This may be a frustrating answer and I do sympathize but please don't shoot the messenger.
Additionally, because this is a hardware problem I'm afraid that the safety of your data is not guaranteed if you do not currently have a back up. However, once the service center extracts the hard drive they should be able to determine if personal data cane be recovered. Due to the instability of these types of issues, I would recommend turning the iMac off and not attempting any other procedures in order to have the best chances.If you continue to have questions on this topic, just reply back to this discussion. I will be more than happy to work with you further. Hopefully, this info will answer your question.
If so please, please remember that when you are asked to give a rating, you are rating my personal service only and not this website itself or your issue.
Thanks and good luck,