"Your Mac stores certain settings in a special memory area even if it is turned off. On Intel-based Macs, this is stored in memory known as NVRAM; on PowerPC-based Macs, this is stored in memory known as PRAM.
Information stored in NVRAM / PRAM includes:
Speaker volume Screen resolution Startup disk selection Recent kernel panic information, if any If you experience issues related to these functions, you may need to reset the NVRAM or PRAM. For example, if your Mac starts up from a startup disk other than the one you've specified in Startup Disk preferences, or if a "question mark" icon appears briefly when your Mac starts up, resetting NVRAM / PRAM may help."
When you turn it on, immediately press Option+Command+P+R and hold it down until it resets and you hear it make the startup chime three times. I’ve rarely seen it resolve anything, but what the heck, it’s free and you might get lucky.