This issue can arise due to following issues:
1. Your computer is set to sleep mode :
In this case your computer will enter sleep mode (by default this is enabled in all computers unless you disable it manually) this will result in machine disconnection and machine will start only when you disconnect the cord and reconnect again.
Here is the link to instructions on how to change the sleep mode:
http://www.ehow.com/how_8163488_disable-sleep-mode-windows-7.html
2. Improper Earthing:
Make sure your PC and Accucraft i250 is connected through same power source and there is proper earthing for the machine.
Accucraft i250 gets disconnected due to communication errors caused by improper earthing.
3. Repetier communications parameters are wrong:
Please find Accucraft Manual attached please have a look at page no 14 through 18 to setup these parameters correctly.
Please select the correct serial port number (this setting changes on pc to pc basis)
You will see the serial port number under device manager settings just like this :
in this case in above image com port value is : com5.