If you have not already, try resetting PRAM
and if that does not help, try resetting SMU (power management)
When you connect it back to power (following the procedure), if you have been using a "power strip" of some type, I would try connecting it directly to a wall outlet, by itself. Initially power it up with everything disconnected, except for the power cord. If you get passed the grey screen, connect only standard USB keyboard and mouse. Use it that way for a while to see how it's running.