This is a pretty generic issue and will probably be hard to diagnose here. You will likely need to try many things. When did this happen and can you think of anything that could have caused it? What type of computer is this?
Here are some general tips
-Do a full shutdown: turn off the pc, remove the power cable to the PSU and hold the power button to fully drain the power from everything.
-Try resetting the CMOS
-Try reinstalling the CPU,Ram,GPU, storage drives and power cables. If any of these are faulty it can cause boot issues.
-If you have a intergraded graphics try using those instead of your graphics card by plugging the HDMI into the motherboard not the GPU.
-If you have any spare parts try swapping them.