You may have just over written the OS. That may be why the splash screens are still the same.
As for it not coming back on when pressing the power button. That is usually caused by installing something on the storage card that need to be installed on the phone.
Any application that resides on the today screen or is needed when the phone first wakes up or is running in the background needs to be installed to the device.
And for the record I actually recommend
DCD ROM's instead of stock. They are generic and plain, without all the fancy stuff that causes issues.