Please check if your application pool has started. If not, and if it goes down after every start of the website, check the Windows Event Logs for the reason, and try to fix it.

Happy DNNing!
Michael

Should I check this on IIS? or where should I check application pool?. Which event log, there are many of them

yes, check IIS manager and ensure, website and app pool are started.

Checking on App pool on IIS, website is up and configured as integrated. Also, checked the windows event logs I saw the following errors for ASP.Net, One appeared on 02/03/22 but error on the website was not present, the second error is a warning that appeared also the same date and in both cases problem wasn't present yet. Any other consideration, Please is very important to have website online, our company needs it

what exactly did you change on production server, before it stopped working?

Actually, I didn't change anything, at least that's what I think, that's because I cloned that VM. After clonning process I worked on cloned VM, I changed things in the machine on this order: 

1. Change IP address
2. Change IIS (Ports related to website and a new website)
3. Upgrade DNN version

After those changes, I noted that in the production VM, any option was in blank. I restarted the VM and the problem appeared. After that i tried to restore productivity VM from it backup but the result was the same, website does no work

Seeing in the error, I noticed that the error says the following: "No sites currently exist for this installation. - 503 Service Unavailable" but there was a site until monday, What happened to my site?. In the cloned computer I don't remember if I deleted the domain and web.config was configured to connect to the real DB (DB is on another server). I was using always localhost but i am not sure if the changes were made in the real DB

did you adjust database connection string in your web.config?
Did you adjust domain address in database table portalAlias?