Assuming you are using the waveshare 3.5in. TFT (or a compatible display), You can switch back to HDMI mode with the following commands:
cd LCD-show/ ./LCD-hdmi Then remove the touchscreen and reboot your Pi.
More details can be found on the waveshare wiki.
If your screen came with a prebuilt SD card image or SD card, you will need to switch to the standard Raspbian image. To allow switching between your regular and touchscreen easier I would suggest burning the standard image to a new card. Full details on downloading and burning the image can be found on the Pi Foundation's download page.
It appears you are running Occidentalis (an Adafruit modified version of Raspbian) therefore it is likely you are also using an Adafruit PiTFT (you could confirm this bt looking at the TFT's board, it should have the adafruit logo and name printed on it. The simplest method of switching back to your Planar monitor would be to download Raspbian as mentioned above, and burning a new SD card.