The most common issues I encounter and this is with the rather limited story given are:

1. The owner didn't plug in the ATX12V connection or the power connection to the video card (remember that no machine details are given so these may not exist.)
2. The owner installed ALL the motherboard standoffs in the new case and they didn't know to only install as many as required. The motherboard shorts out to the case and if they are lucky, removing the extra post brings it back but no warranty is ever offered and in fact as a shop owner you would write a note on the ticket stating the find and voiding any future or present warranty on the work and machine.
Bob