I used to have similar problem but it was actually for PC operating system and specific program. I solved it with running [link|https://www.trialresetter.com/|trial resetter] which is actually a tool which reset any specific program back to is defaults. I am not sure if there are any available online for iMac but you can search of it. I didn’t understand your question clearly so if you want to restore your iMac without any discs I prefer following the steps mentioned above by the Gaetan.