Is my T420 screwed?

My T420 display has stopped working. It's just blank.

-> If I hook up my laptop to an external display, and then press Fn+F7 as appropriate, I can use my computer and see what's happening on the external display.
-> The backlight seems to work: if I press Fn+Home and Fn+End to adjust brightness, the monitor changes to lighter/darker shades of gray.
-> The problem is not isolated to Windows: the POST screen doesn't show either.
-> Occasionally (but rarely) after I restart the computer, the monitor magically starts working again. Then it keeps working until the laptop turns off. (I've had this problem all week and have restarted the computer dozens of times. There's no apparent trend to whether a restart will get the laptop working again-- e.g. it's not related to heat level, because even after the computer has been off for 24 hours, it doesn't work when I turn it on.)

When using the laptop on the external display, I notice that Windows doesn't detect the laptop monitor properly, thinking that it has a maximum resolution of 1024x768 (instead of 1600x900).

What could this be? My warranty expired recently but I might be able to buy some more.

 

could be a loose screen ribbon, if on an external monitor and and doesn't have problems after fn+f7 then it's not the gpu.
either you open it up to check, have a professional laptop repair open and up to check (since out of warranty), extend your warranty and have it repaired.

as for screwed, not really it's easy to replace the screen (if the problem is there), but if it's the motherboard then there's more work involved.

but it definitely sounds fixable.

 

I will go with Geekz advice and check the ribbon cable at the back. Thankfully it is easy to access the LCD cable on the T420 since the LCD bezel surround is easy to pry off with your hands. If you need more information then you can use the hardware maintenance manual.

If the cable seems OK then with the external monitor connected turn on the ThinkPad and keep tapping F1 and it should go straight to the BIOS. Go to Config, then Display and check that it says ThinkPad LCD as the default option and not Analog (VGA). If that's fine then lastly I would do a check on the video drivers, if you had updated the video drivers recently then try performing a rollback using the Device Manager and see how it performs from there.