No contacts available - no landing request possible

I was travelling with several hops in order to reach the final destination and in each system I stopped in during my travel I scanned the system in order to collect data (for the first time I was using my scanner).

But then it turned out that I was not able any more to request landing on any station. The stations were targeted, I was not in supercruise, I was located 1000-200 metres away from the stations respectively and my weapons have been retracted, but in the "Contacts" tab nothing was listed, so I was not able to request landing.

Somewhere I've read I'd have to enable my radar, but I don't know how to do that. Was is my radar? Or better: How can one turn on/off the radar? I didn't find anything like that in the command options.

What's going on there?

I am certain that your problem is caused by your Sensors being disabled in the Modules tab of the right hand interface screen. I have tested this myself and got the same results as you.

Open the right hand interface screen, change to the Modules tab and scroll down to locate the status of your Sensors.

If the text for the Sensors entry is yellow, then the check-box on the for them will be unchecked. Toggle it so that it is checked and you should get your sensors back.

If the text for the Sensors entry is red, the check-box will have a line through it as it is disabled due to your Power Plant not providing enough power for all your modules. In this case, what you need to do is change the priorities of each module so that they are available when you need them to be.

Highlight a module, and use left and right to change its priority. A value of 1 means highest priority. I suggest setting all modules that you want to be available always (Thrusters, Shield Generator, Scanner etc) to priority 1. All weapons/combat modules (Kill Warrant Scanners, Cargo Scanners, Point Defence) to 2 and everything else to 3.

With this configuration (assuming your Power Plant can power your weapons when they are deployed) you should be fine. When you deploy your weapons, if you don't have enough power, things like the Cargo Scoop, Fuel Scoop and Frame Shift Drive all shut down but you can't/won't want to use these when fighting anyway.

Note: If a module doesn't consume power, you can't change its priority (such as the Power Plant, Discovery Scanners and Cargo Racks).