What logs are worth looking at to debug issues when mining? For example I had a problem with a miner I just put together. It was mining pretty well for about three days then I had to shutdown to reroute some power cables. When I booted back up two of my four GPUs weren't hashing at all and the other two were running at like 30%. In a situation like this where should I start debugging? I know my way around a command line but by no means a linux expert. Any general advice would be greatly appreciated.

Thanks
PC