When I first connected my Prusa 3 build to power, the x axis would move a short distance and the y axis would move over and back only about 10mm. But now there is no movement at all when I power it up. What could the problem be? Or is it supposed to move axially when powered up?

Shouldn't move at all when you just power up the printer.
As I don't know where your at with the printer it's hard to comment. But, have you compiled and uploaded the firmware?

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Waitaki 3D Printer

Hi my name s Alessandro and im trying to build a prusa i2, i have finished the structure and the problems are of software.
I used 3 different firmwares: Marlin, MarlinKimbra and Sprinter.In all of them there is the same problem: the motors doesn t work.
For me the problems could be 2:
1) on the software i wrote that i have 1 extruder and 2 thermistors (extruder + bed), but on pronterface appears an other extruder and his temperature is 0°. Probably the motors doesn t work for that.
2)the power supply (12V,400W) isn t right for my prusa i2. Maybe there isn t enough current.
Thank you so much for the answers!
P.S.
When i try (on pronterface) to move the motors they do not even make noise.

I HOPE that i wrote in the right section of the forum and in correct english.