Whites discontinued the DFX in 2012. Whites went out of business last year and Garrett purchased their existing stock, current patents and the rights to their trademark, etc. Most of the Whites detectors I have owned were weak in the battery/power connection department. I agree with everyone so far....check all possible battery/power connections. Luckily the main circuit board is easy to get to as are the power connections an battery holder connections. If you have a second battery holder, try it too. If that doesn't solve it you might have a bad display/touchpad.
The DFX simultaneous multifrequency operation technology is very rudimentary. The display, especially the signagraph, were fantastic. The only comparable detectors from Whites are the V3 and V3i. The only other simultaneous multi frequency detectors made today are the Garrett Ace Apex (pretty good from what I have experienced) the Minelab Vanquish series which are excellent entry level detectors that detect way above their price, the Minelab Equinox 600 and 800 which are excellent intermediate detectors and the Minelab CTX 3030 and E-Trac which are still in production.