The green light comes on and stays on, but the screen is black and no sound. Main board or Power board?

Sounds like back-light inverter fault. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

have you checked for dry joints or blown capacitors , with a power surge depending what kind voltage or amperage the circuit received it might have even blown screen connect it will certainly fry a lot of components ,have you looked for holes in capacitors ,resistors or burnt tracks between components . that's about all I can think of to check also some power panels have built in fuses to stop surges if you could obtain a circuit diagram that should help you also you could google your query and see if you get any assistance it costs nothing and you could get your TV up and running again . All the best .

Fault with main board. Damaged component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/

replace the power supply board.

The power supply regulator board to your TV is now working fine, and has no problems at all, as, there is standby light. Check all the other voltages comes out from the power board, to main board. If all of it found OK, the fault should be at its main board.
About blinking of standby/power light;
These blinks (flashes) are fault indication. By the number of blinks occur at one time switch ON, you can get an idea about the faulty circuit stage inside the TV. Count the blinks occur at one time switch ON, carefully. Do it once again, and make sure about the number of blinks. If the flashes are continious, non-stop, the main system control IC should be suspected. If you wish to get more details, check the site linked here. It has blinking codes detail to many brand TVs. Pull up older posts. Check the site in 'Mosaic' view. It will make search easy. http://electronicshelponline.blogspot.com/

problem is probably the main board since both audio and video are affected.

It sounds that the main digital board to this TV is faulty. Contact any service technician.

There is some fault with its digital processor board [main board] Contact any experienced service technician. This is not a user repairable one. If you are not familiar with electronics, do not attempt to repair. Whether you suffer fatal electrical shock! Instead, contact the nearest service center.

Can I buy the tv for parts off you?