Laptop - black(ish) screen unless connected to second monitor

Can anyone offer any suggestions to solve an issue I've had for a while with my laptop screen ?

If I start up my HP Envy laptop as normal, the screen doesn't work correctly - it's not a black screen as such, but black with varying degrees of grey/white stripes.

A brute force solution to this, that I read somewhere, is to hold down the power key until the laptop powers off, then to turn back on, but hold down the power key as it's powering up so that it doesn't fully come on. After doing this two or three times, the screen will look slightly different as it's powering up (instead of black with grey stripes, it will be 'cloudy' black - you have to notice it quickly, and release the power button, after which it will powerup perfectly fine, screen on OK.

None, of this is necessary if a second monitor (or anything - ie my TV) is connected via an HDMI cable - if this is the case the laptop will power up with the screen working fine. (If I wish, I can then disconnect the second monitor, and the laptop screen remains OK.

Any way that I can fix this ? Thanks in advance.

Hi Bruce - many thanks for your reply.

The screen is abnormal right from the point of turning on; the white striped 'interference' will then increase depending on what's happening on the desktop 'behind' the fault (ie when you first switch on, the screen is mainly black with a bit of grey banding, then as the system opens up, and you arrive at the desktop there will be more white striping).

I think that the effect is just the same going into BIOS - trying what I think is the BIOS key for my laptop has no visible effect, but I can't tell if I'm in BIOS, behind the faulty screen, or if I'm not in BIOS at all.

When a second monitor is connected, startup is just the same, with the faulty banding, until you get to the point just before the lockscreen/sign-in screen appears - the sequence will follow the same as the process described in my first message after 2-3 attempts of holding down the power key while powering up : black screen (proper black) with white stripey interference, then the screen switches (I don't think I actually get a windows logo - don't think I ever have) to 'cloudy' black, usually with a brighter 'frame' around the edge, the the cursor appears with the spinning circle, very briefly followed by the lockscreen - I have mine set to not require sign in, so that very quickly disappears and the desktop fires up totally normally.

Don't know that description points in any particular direction ?