The Lnb may have started to drift , try doing the below.
S7070 LNB Drift
To correct for an LNB that has drifted in frequency the steps are as follows
Press the menu button on the remote to bring up the main menu.
Select Installation and Press Ok
Select Satellite Setup and press OK
Select LNB No and press the right arrow until 8 is displayed
Select LNB Low Frequency and using the number keys enter 10745
Select LNB High Frequency and using the number keys enter 10745
Select 22Khz and press the right arrow to select Off
Press OK to start scanning
Select "Yes" and press OK
Once the scan process is completed you will be returned to the Installation menu. Press the Exit button twice to return to normal viewing.
Check your signal strength in the Tuning Menu too. My Sat receiver shows around 56% signal and 40 - 50% quality an my 9 yr old Dish and LNB. A Sky Tech told me 50 -60% is considered about normal.
Id be looking at replacing the LNB.
If the channels disappeared suddenly rather than deteriorated over time it could be a STB failure. I'd check the setup with a known working box first before replacing the LNB.
Or try your STB on another known good Dish Antenna (with the same LNB frequensy).