I have a Pi connected to a Polaroid tv by HDMI cable in order to create a "Dakboard".
Now when initially starting the project the TV was acknowledging a signal coming from the HDMI port (ie the no signal error message disappeared) but the screen remained black. This was easily solved by uncommenting the "hdmi_force_hotplug=1" and "hdmi_drive=2" lines in the boot/config.txt file, the pi was now displaying on the tv.
I have now however added a crontab job (following the tutorial linked below) to turn the display off at a certain time then back on at a certain time by enabling and disabling the HDMI port. The port is disabled as desired at the correct time therefore leaving me with a "no signal" display on the tv, however when it's time to enable the port again the "no signal" message disappears but nothing is displayed, it's just a black screen.
My guess is that this is the same issue as when I initially started and that I somehow need to add the "hdmi_force_hotplug=1" and "hdmi_drive=2" into the crontab run .sh file.
If anyone has any suggestions I'd be keen to hear them. The tutorial I have built this using is linked here;