Monitor Turns Off when loading TFC multiplayer game

I recently switched to Open GL video mode for half-life, and when I come to play the TFC MOD, and attempt to connect to a server, the monitor turns off. I accidentally turned the monitor off when loading a game, and it would not go on then, and even after i restart the PC the monitor is set to turn off(standby)every time i load a game. It doesn’t happen when I use software mode.