@Chazza234
Hi, you mean the signal LED off? if so, it means there is no wireless connection established, so the extender has disconnected from your main router now.
Please verify if your main router WiFi still works fine, if so, please reset the extender to factory defaults, check if you will be able to see and connect the default TP-Link WiFi of the RE450. If so, you could set up the extender again to connect your main router wirelessly.
Via Tether App: https://www.tp-link.com/support/faq/1373/
Via Web interface: https://www.tp-link.com/support/faq/1400/
By the way, you mentioned that the power LED flashes occasionally, usually power LED will flash when the extender is starting up or firmware is being upgraded, please make sure it is plugged into wall sockets firmly. If it still flashes when nothing happens and even affect wireless connections, there might be some issue with the extender, please confirm if there is anything special happened before the problem started, such as power outage, firmware update etc..