Can you help with the folder with question mark logo?

Hi, I am getting a folder with a question mark in it. I turned on my iMac after a long time. It started up but it froze on Home Screen with wifi option on.

I couldn’t click or press any buttons so I did a hard reset by pressing the power button and after that I started getting that logo with a folder and question mark in it.

Only thing that works is if I hold down the alt/option key are startup but it only shows a mouse cursor.

Help. I do not want to lose my data on it and also I don’t know what operating system it’s running on and don’t have a backup.

I have another iMac that was connected to it via Ethernet cable long time ago. I have also heard of an option by running it through FireWire with the other iMac but I don’t have FireWire cable and don’t know if that would work or not