Blank screen after Grub (cannot even open terminal to try to fix)

System: Ubuntu 18.04 Bionic dual boot with windows which works fine previously for two years.

Problem: After selecting "Ubuntu" in Grub menu, screen is blank, no login screen, nothing, I can't even open terminal with shortcuts CTRL-ALT-F5/F1.

Possible couse of problem: I'm pretty sure its Nvidia related problem (se description) and I need to reinstal its driver, but cannot acces terminal, so Im stuck.

Description:

1. I updated BIOS (as recommended by Lenovo update system).
2. Everything works fine, only my second sreen didn't work.
3. I tried to change Nvidia driver version (in Software & Updates application) from version 450 (which works for me until now) to recommended version 470.
4. So I choose Nvidia version 470 (tested), click on apply changes and changing driver process started. During the process, popup window appears "Configuring Secure boot" with notification that my BIOS use secure boot, and that I should choose password during the process, etc.
5. I remmembered recommendations that i should disable secure boot after upgrading BIOS in dual boot, so I decided to try that before that process (of changing driver version) finishes.
6. But I cannot cancel process, becouse only option on popup window is "Next". So I (obviously wrongly) restarted system and enter to BIOS
7. I disable secure boot in BIOS
8. Started the system, Grub menu appears, choose Ubuntu.. and only I get is BLANK SCREEN (only short splash screen "Legion Lenovo" appears and disappears).

That's why I'm pretty sure the problem is Nvidia driver (process of changing which I interrupted). So I'm sure that reinstaling Nvidia driver will fix this, but I have blank screen, and cannot do anything.

I read posts on forums, but all of them suggests use of terminal, but I cannot acces it :/

Suggestion to click "e" on Grub menu, and change "quiet splash" to "nomodeset" doesn't help.

Of course, I can open windows system (dual boot), and there everything works fine.

Thanks for any suggestion

To answer my own question.

Solution which works for me (as expected, problem was Nvidia driver)

1. Enter BIOS - Configuration - Graphic device.. Two options: Discrete graphics and swhitchable graphics. Discrete graphics were selected which means that system will use Nvidia discrete graphic. But mine was not properly installed (I interrupted process, see description above). That's why I got blank screen.

2. Choose "Switchable graphics", click Enter

3. Make sure that Secure boot is disabled under Security menu (there are some problems reported with Nvidia drivers when it is enabled in dual boot) Navigate to Exit menu and save changes.

4. Now I was able to start Ubuntu with normal screen visible (and all applications as normal). Open "Sofwtare & Updates", than "Additional drivers", Nvidia driver which are selected is one which is not installed properly (see description in question) (version 470). So I choose another one (version 460). Aply changes. Now I have regularly installed Nvidia driver.

5. Now I can bring back Dicrete graphich option in BIOS: Open BIOS - Configuration - Graphic device - choose Discrete graphics, save changes on exit.

6. Restart system - everything works fine, even my original problem (second monitor blank) is fixed. Actually, I could solved it easily just disabling secure boot (see 3.) after upgrading BIOS, but instead i tried to change Nvidia driver which couses further problems.

7. Nvidia driver version 460 gives some flickering on my screen (I've experienced that before and I know that changing Nvidia driver will solve it) so again open Software & Updates - Additional drivers - and I choose recommended version 470 Nvidia driver.

8. Restarting system, and everything works fine: built in display and second monitor.

This "solution" works for me. I'm obviously ordinary user, not an expert, so take everything here with caution.