Try an engineers reboot:
1. Switch the power off at the mains
2. Put and keep your finger on the channel up button (arrow up button on some boxes) on the front of the STB.
3. Switch the mains power back on keeping your finger on the channel up button.
4. Keep your finger there until the standby light comes on permanently (you may notice it flicker on and off.. wait until the light is sturdily on) or a channel number is displayed.
In some cases, this will bring the channels back temporarily until the engineer arrives. In others it will bring a 0 to the front screen of the box, in which case you will need to wait until the engineer arrives. Faults can send signals down to fix 0 on the box (which will take two hours to clear), but the likelyhood is, there is a problem with the box itself and the box wont respond to signals.
If this trick works, make sure there is power going to the STB until the engineer arrives. If you switch it off, it may go back to TUNE agin. If it works or not, please keep the engineer appointment. TUNE problems will come back!
Hope that helps
