I have a big problem:
Last week, I installed Kali Linux (boot)on my old computer with an iso installer which is at my school, and it worked fine.
But two days later (I hadn’t checked my computer during those two days), when I turned it on, I got a black screen with a blinking cursor.
So I reinstalled Kali Linux, and it worked again. However, after two days, the same problem happened again.
It sounds like your boot sector is not being created properly.
Is Kali the only OS on the computer?
Did you disable secure boot etc?
Has it ever booted directly from the computer from a ‘cold boot’ after being installed ?
(without the live ISO attached)
Is this a standard hard disk install or an SSD?
some SSD based systems don’t have proper BIOS support to boot properly with Linux, do a search for a BIOS update for your computer.
Need more information to help you get Kali up and running, the computer make and model at least.
Kali is the only os in the computer, it has an old hdd (500go) and I didn’t find any bios update, the secure boot is disabled and I did the installation with my usb key (who was flashed with the iso) and after the installation, I boot without the usb and it worked , after 1-2 days it show me black screen with blinking cursor.
(I don’t know the computer model so I will verify tomorrow and I will tell you
If it worked for a few days with only Kali installed to the hard disk, and nothing else attached, then it did install correctly, so it sounds like your grub was corrupted somehow.
Do you see the Kali option screen at boot up, where you can choose safe mode etc?
Next time you install it, from first reboot, update the system;
sudo apt update && sudo apt full-upgrade -y
I don’t see the menu option, I turn on my pc and then i have black screen with the cursor and nothing else
so basically as soon as you ‘cold boot’ you have no Kali working?
That is likely a Grub error, If you previously had windows on this computer, then before you try installing Kali again, from the Kali live ISO open GParted and delete each and every partition you find on the target disk.
Then format the disk as ext4 and the Kali installer should set it up correctly for you, including your boot sector.
Ok thanks , I will try it
Just to point out, the OP said this is an old computer in “school”.
If it’s connected to the school network, maybe they reimage machines on the network nightly, or regularly.
Also, has he any way of knowing that someone has not messed with it when he’s not there? There might be someone else on a forum somewhere complaining that every time they go into school kali linux has appeared on their computer out of nowhere. 
The device is connected to the school network, but it’s not shared, and I disconnect WiFi after each session — so if someone did mess with it, that would be impressive and funny.
I’ve already gone through the usual checks.
Is there an ethernet cable plugged in?
A lot of schools reimage their machines nightly so that there is no inappropriate content left on them. Are you 100% sure they don’t do that?
Do you have permission to install kali on a school computer?
That’s a good point — I hadn’t thought about reimaging. I do have permission to install Kali Linux, and the computer is connected via Ethernet during the session (I unplug it afterward). I’ll make sure to check if any reimaging happens overnight, just in case.
By the way, I’m curious.how does reimaging work in school environments? I’ve heard that some systems reset every night, but I’m not sure how that’s set up or triggered
There’s lots of ways it is done. Why don’t you ask your schools IT department?
Ok I will ask them 