Looks like a damaged Startup disk? Please, please help!

Hello guys,

My iMac (27" late 2015 running OS Mojave) froze last night and had to force shutdown. When I powered it back on, it's stuck on a blank black screen. Someone suggested an NVRAM reset - took awhile to load but it did work for awhile. However...

First problem: Files on desktop and downloads folder seem lost. I do have my desktop files on iCloud; but how do I restore everything back to its previous state? It's the same user account (didn't get deleted).

Second problem: Every time I turn the machine on, the startup screen shows a flashing folder with question mark. Did NVRAM reset again but didn't help. Pressing the Option/Alt key doesn't reveal my Macintosh HD and only shows my External harddisk with one partition. I tried Cmd+R; but stuck at the WiFi screen because it wouldn't allow me to type into the password field. Connecting with ethernet as we speak; but it doesn't seem to make a difference.

I'm desperate to get things fixed and been on it for the whole day...can someone please kindly guide me in the right direction? Many thanks in advance! I really appreciate it :)

Kind regards,

Bryan