Warm reboot comes up in black screen. Cold reboot sometimes works

Toshiba L775 laptop. Sometimes boots up fine. Sometimes boots up to black screen. If one taps the power button, to put the machine into sleep, then bring it back up, that usually fixes it (not always). Hard drive activity light shows the machine is actually booting up / doing its thing.

Cold boot is slightly more likely to boot up normally, while warm boot is slightly more likely to boot to black.

There was a similar question, but the answer does not apply here. A secondary monitor attached to the lap reveals precisely the same results.

Windows 10 (alas, but what’s one to do?). Most updates in place (latest “round up” doesn’t install — backs itself out). Have scrubbed for malware (EST, MBam, F-Secure, bit defender rescue disk), so I doubt that’s the problem.

Help!