I installed Ubuntu server from USB stick, installation went fine, but when i'm trying to boot it nothing happens, no error messages, no blinking cursor, monitor just reports "no signal". The weird thing is I left it for a few minutes, pressed enter and it loaded just fine. And now I can't even repeat that trick. So I'm totally confused, any ideas?
You can try any of the solutions posted here in this AskUbuntu post about booting to black screen issues.
Test nomodeset during boot by editing the kernel line in GRUB2, this will prevent the kernel from messing with the video mode and use the BIOS video settings in place during boot.
You can do so by pressing Shift right after your BIOS post, once GRUB loads go to the relevant boot line and press e, find and replace
Another possibility is that GRUB is trying to use a mode that your screen does not support, to edit that you need to at least have some access to the server (SSH would help!) and edit the file