Hi there,
I installed ubuntu mate 18.04.2 on a new PC (lenovo ideacentre). This is a fresh install and Mate is the only OS on the PC.
When I switch on my computer I rapidly have the green login screen asking for my pasword. my problem is that if I don't give my password within 5 min, it turns to a black screen and I have to press a key to get back to the login screen.
Moreover, today I gave my password rapidly, then my PC logged in my session and automatically launched thunderbird. that's OK except that it the turned to a black screen (I did not do anything before it happenned) and did not returned to my sesion when I moved the mouse or pressed a key: I had to restart my PC to be able to work.
Precision the nergy manager is set on "never" for screensaver or any "turn off" options.
Do you have any explanations / solutions to manage this ?
Thank you
Hello mpruffin
My first act would be to download the ISO again (Torrent), run the checksum, transfer to usb or dvd installation medium and do a fresh install.
If the problem persists come back again. I am not a technical expert, but our community has a lot of knowledgable people. 
EDIT (02.04.2019)
This post UM 18.04.2 logs out instead of suspend and destroys unsaved documents
says that 18.04.1 and 18.04.2 have different kernels. It may be worth trying 18.04.1.
Thank you alpine john, but it's already a fresh install, and in fact the problem already existed with me previous (old) computer and mate 16.04, with which I tried a complete reinstall.
I thought the PC was responsible for it but obviously it's not the case.
Could it be related to graphics management specificities within the pc (no graphic card but intel graphic HD processor manager) ?
The fact that's it's already a fresh install is irrelevant. Sometimes weird, unnoticeable things happen during installation, no matter the OS, that only trying again can solve. As it doesn't really take too long, I think that @ alpinejohn's suggestions are worth exploring -- especially as it's been recently done and (hopefully) you haven't had as much of a chance load it up with additional apps and data.
Hopefully, it's just a matter of your Lenovo being temperamental. I know that all of the ones I've had over the years have been. But would I give them up? Hell no! I love 'em.