Yea it depends on what's wrong hey.
If they go on, but don't hash and you can't access the Dashboard, but can ping it = you need to take the SD card out and give it a set IP.
If they go on, but don't hash and you can't access the Dashboard and can NOT ping it = you need to reflash the SD card or replace it with a new one.
If they go on and all 3 lights light up during startup you might be in luck - boards are fine and the problem lies elsewhere.
If they go on, but no blue lights light up during startup - there's something wrong with the board.

I'm currently sitting with one hash board which I cannot get to work for the life of me. I've fixed a few but this one... Disassembled, cleaned, went through it with a magnifying glass, applied new heatpaste.
I just can't get the blue light to go on during startup. It goes solid when the other 2 boards hash (tho itself doesn't hash).

If they don't go on at all... Something is most likely fried - should be able to see it when disassembling.
Eh, but it's a pain in the arse to send back with fees - and with the current profitability, I donno if it's worth the money sending back :/