Ok, so here's what happened.
After my last post I went on with ubuntu desktop demo mode, and checked all partitions and files.
1. i found out that the datastore was 100% OK, this was a relief.
2. I restarted the server and installed esxi on a usb. and started it up,
3. The datastore was recognized, so I started to backup all the vm directories to an external drive. Because I didnot want to risk it also getting corrupted. this took few days, cause few of vm's were quiet big.
4. while restarting the server suddenly I noted there is a new disk, in the boot list. sometimes it showed and sometimes it didnot. this is how I found out that there was an SD Card installed on the Motherboard, and the Esxi (the original crashed one) was installed on it.
5. I started the ubuntu desktop demo, went to the sd card, second partition, and found there a file called state.tgz opened it and found out that this file contains all the information about the host, host settings, vm machines etc... including the license.
6. I copied it to a usb, then I went to check why this SD card didnot start properly, I tried putting there the files that were shown in the boot error. but that didnot help, because some of the files in the error didnot exist on sd card but also didnot exist in the usb with good working esxi, so after many many tryouts I left the idea of repairing it. and moved on to new SD card.
7. I tried for like 2 hours to install a new SD card with esxi on it, that didnot work, then I tried installing esxi from usb to Esxi, same problem, after installation starts, at 5% it says unable to write etc... While I know the sd card is 100%, it's new.
8. I removed the SD card. connected a usb with esxi on it and installed fresh esxi on it and booted the server. then after server booted and all was working, of course I was missing all the settings including Vlan / networks etc...
9. booted with Ubuntu again, copied the state.tgz file from backup sub to the working esxi usb. and started up. Host was completely restored to original condition. All settings, all Vm's all 100% working now.
so I guess then, the reason was that my originally installed SD card of 8GB from sandisk got corrupted, that's why the server was unable to start.
I also found out that there is a file state.tgz which contains all settings of the host and vm's. wich I didnot read anywhere, and didnot know before, using esxi's for more then 10 years :smileyhappy:
so for now this is solved.
thanks to everyone trying to help. I hope you guys never experiance this, it's a big stress... but we came out dry this time.