Displays New monitor DVI or HDMI no signal

I just bought a new asus monitor 2 weeks ago. everything was fine until yesterday. The monitor knows when you turn on a computer hooked up to it and turns on by itself. So yesterday when i tried rebooting my computer it said "dvi no signal" i tried to to change the input again to dvi and the buttons did not respond and i couldn't event turn off my monitor so i unplugged the power cable and plugged back again and this time it did recognize it. It also happens when i'm using my other computer with the HDMI cable! both cables are hooked up to the monitor but i don't use both computers at the same time! can anyone help me fix this issue? thanks!!!