If everything goes dead when the key is turned to Start this will nearly always be an issue with the battery or connection at the battery.
The dash equipment only requires a small amount of power to turn on (around 1 amp typically tops). However when you turn the key to Start, this triggers the starter which can be upwards of 100A draw or even more depending on temperature, load against engine at that position, etc. When this occurs and the battery is depleted or connection poor, it immediately floors the battery killing the other items that were relying on that minimal amount of power the battery was able to provide.
Before doing anything else, check your cable connections. This can often just be a loose cable or corroded terminal. If you can move either terminal by hand, it is too loose and requires tightening. If there is any sign of corrosion the terminals will need to be disassembled from the cables and the contact points cleaned.
If the cables are tight and clean, you will need to have the battery tested to check it's reserve power is within specification. At this point you will likely find the battery out of spec. Keep in mind that the test results are relative. For example the battery can test out at 250CCA, however still have the symptoms you experience as the battery rating is 700CCA, etc. A high number is only good in relation to the specification.
In very rare cases we have seen these ignition switches
/housings fail so that when you turn the key to Start things go dead as it turns past the posiiton, however this is highly unlikely in your case for two reasons... first, when this occurs you will normally hear the starter engage as you pass the position into a dead zone in the switch. Second, when this is the case power will immediately be restored when you release the key (and likely hear the starter engage again as it passes it's position.
Verify your battery and connections, it is nearly a sure thing you will have an issue there.