Z-axis drops below bed at start of print

[Mcgee1505]

Hi all, I am a newbie on this forum and cannot find a thread to answer this issue.

I have built my Prusa i3 and followed the setup for the Home and bed levelling process. I tested the home function and it works great. All the motors are working fine. I have pre-set the nozzel temp to 200 and the bed to 70. I load my print file in to Ultimaker and send to printer, the LCD shows printing but as soon as the motors start moving, the Z-axis drops below the home setting and drags across the bed. How can I stop this and make it print correctly.