On the monitor, make sure to change between the input sources, most monitors will have the feature for selecting the input that the monitor is receiving a signal from. Make sure you have selected the proper input.
For notebooks, remove the battery and power cable, hold the power button for 10 seconds then wait an hour. Reconnect the battery and power then try powering it on again. Try the display mode shortcut keys (FN+F# key, usually)
Try reseating the RAM, test one module at a time in different slots.
Try reseating the GPU, refer to your motherboard manual for instructions. The general instructions are:
Turn system off
Remove Graphics Card
Press power button (without power plugged in)
Reinstall Graphics Card (you can try different compatible ports)
Check for recurring issues
Try reseating the CMOS Lithium battery, which might be located underneat the graphics card or near the CPU.