Transfered working PC into new case now it won't power on!

Here are the very first two steps your Really should do.

1. Breadboard check
Remove all the system from your new case. Choose a clean dry surface to set up your stuff (that's why the term "breadboard"). Assemble only the minimum parts to test whether they work: the mobo, CPU and its cooler, one stick of RAM, PSU connected to mobo, keyboard connected to mobo, video output connected to monitor. Make sure you have plugged power from the PSU into both the main mobo input and the CPU input connectors. IF your video card needs power directly to it from the PSU, connect that. Turn on the monitor and the PSU's rear main power switch. Now use the "paper clip test" procedure of shorting together briefly the two pins of the mobo's Front Panel header that are labeled for the POWER switch. Only a brief shorting of those two pins is necessary - could use a paper clip or a screwdriver - and then remove the connection. That is sufficient to start up a system that is OK. This will test what you forgot to - does your old system still work OK? If it does start up but halt because it cannot boot from no HDD, your system is good so far. Now in sequence, repeat a series of steps. Shut down. Add one more component, then start up again. This tests that component, too. Proceed to the next, until your complete system is connected and working on the table top.

2. Stand-Offs Check
Whether you have complete success with that or not, do this step also. The new case has a pattern of holes drilled and tapped into the base plate, and many of them will have stand-offs pre-installed in them. These typically are short (3/8" high) metal pieces with a tapped hole drilled into the top, and a threaded short end sticking out the bottom. The threaded end screws into a hole in the case base plate; the hole on top accepts a screw. These units are used to mount the mobo to the case. There are a few "standard" mounting hole patterns, and the case itself has enough pre-drilled holes to adjust for all of these. BUT what is VITALLY important is this: the Stand-offs must be installed in base plate holes ONLY under where the mobo has a mounting hole. Often there are 9 such holes in the mobo, so look at yours and observe exactly where they are. The mobo is designed to be supported and Grounded to the case at each of those holes - note that each mobo mounting holes has a few little metal fingers around it to make contact with the screw you put in there. But the mobo must NEVER contact the case and Ground at any other location. So the stand-offs establish a clearance space between the case base plate and the bottom of the mobo, as well as providing the attachment points for the screws. Ideally there will be one stand-off under each mounting hole, but there must NEVER be a stand-off under the mobo where there is no mounting hole. So, compare the pre-installed stand-offs with the mobo holes and make SURE they match up. Move any stand-offs in the wrong place.