iMac not detecting external displays

I have a mid-2010 iMac which, for a long time, ran with a Samsung TV as a double monitor through the mini displayport (via a VGA adapter). This past Christmas, I received a Yiynova MSP19U artist's tablet, which also uses a VGA connector. After installing the drivers and everything, it connected and worked just fine. However, every time the iMac would go to sleep (or the monitor was disconnected and reconnected), it would stop recognizing the external monitor. I would have to shut the whole thing down and reset the PRAM. Usually, after doing that a few times, unplugging and replugging, it would recognize it again. However, yesterday, it disconnected, and now it won't reconnect. I've cleared the PRAM multiple times, I've cleared the SMC, I've unplugged both devices and all peripherals and then turned them all on together and separately. I've also tried to connect another monitor that uses VGA, but still, nothing is connecting. I've connected the tablet to my wife's PC, and it works just fine. I'm at my wit's end with this whole thing. Does anyone have any other potential solutions, or do I just need to take it to the Genius Bar?

Another issue may be that after it disconnected yesterday, it needed a software update restart, which I allowed (an update for Mountain Lion, which is what I'm running on the computer). I don't know if that would've affected anything.

Thanks for the help.