Unsolved E4.5 suddenly has black screen on startup

[Tominto]

I have the latest version of ubports for my e4.5 and have been using it with no problems. However last week, when I went to restart it after putting in a sim I use for travelling, I ended up with a black screen after the ubuntu boot screen. The BQ flashscreen comes on fine, and then the ubuntu boot screen comes on very briefly, followed by a black backlit screen. I've tried various combinations of power plus up volume button, down volume button, and just pressing the power button with the same result. Nothing seems to work. Any ideas would be greatly appreciated.

[domubpkm]

@Tominto For the smartphone to be able to attempt to restart, you need to press the key(s) for about 15 seconds if you didn't try that.

[arubislander]

@Tominto did you remove the sim card and then try rebooting?