My procedure for what you describe (No signals at all, on any channel) would be:
1. Check the fitting of the F-connectors for good contact of the inner and outer cores with no shorting and a correctly size connector for the cable width.
2. Check no channels are being received – a good way to do this is a non-freesat FTA tune.
3. Put unit in standby swap LNB1 and LNB2 connection retest 2.
4. Check for water ingress – normally you wound loose a block for frequencies and this would progressively expand.
5. Suspect the LNB.