I am trying to set up my Raspberry Pi in headless mode (i.e., without a monitor). I have installed the Raspbian Wheezy OS on my SD card. Initially when I powered it on with the ethernet cable connected to my laptop and ran the Advanced IP Scanner, it successfully detected the Pi and I was able to access the Pi's terminal with PuTTY using the Pi's IP. I logged into the device.
When I tried to access the Pi later, doing everything in the same manner as I described above, the Advanced IP Scanner was not able to detect the Pi.
The Pi is powered on and connected to the laptop via the ethernet cable. What am I doing wrong?