I have an older Toshiba laptop with Windows XP which will not connect to an external monitor. When I attach an external monitor it will come on initially when the computer is turned on but then go to the "monitor is OK, check your cables message" and only the laptop display worked. Now the laptop monitor is completely gone and the same thing happens - external monitor displays at first and then goes to an error screen.
1, Go into control panel, and click on DISPLAY in the control panel. Does the computer detect your external monitor? Can you change the resolution of it? Can you "Extend the Display" to your external monitor? You should be able to manipulate settings here inside this screen if it will even pick up your external monitor.
2. Fn + F5 keys - toshibas have function keys to allow you to connect an external monitor. try using these two keys to activate the external.
What is the model number of your computer? I want to find you the most updated video drivers to install, this may help repair the problem. ALSO - do you have another cable that you can use?