It sounds like you have a Mitel 3300 with a CXi controller, meaning one with an embedded 16-port POE switch? And the phones are not booting, just lighting up and showing no additional information in the display? Sure sounds like a hardware failure in the controller itself, and switching to VDSL was just a coincidence, however, although coincidences like this happen, it is extremely rare, so troubleshooting with someone who understands networking and VoIP would be useful. Try connecting a phone with a POE injector or to a POE switch connected to a switch on the network (off the LAN port of the Mitel somewhere), but to troubleshoot this we would need a lot more info, starting with what the phones show in the display, if anything.
A competent Mitel tech can reset the password and get into the controller and see what is going on, I would suggest contacting a Mitel vendor in your area as soon as possible.