UPDATED:
PROBLEM:
Converter worked fine, but then now shows only a blue or black screen.
SOLUTION:
Some converters, especially the cheap ones, are prone to failure. either the internal power transformer was of bad quality and therefore opened a winding which results in no power and a dead box (no LED on front) or the box is in a state of 'confusion' and before assuming the worst, try unplugging the box, let it sit for 10 min, and then plug it back in. this 'reboots' the box's internal memory and may bring it back to life. also verify that you didn't accidentally turn the TV channel to something other than 3/4 or if the box is hooked to a VCR, make sure it's also still on Channel 3/4 (or line input if using A/V cables) and turned on and in 'VCR mode'.
PROBLEM:
HELP!
DTV was fine until just after June 12th, now i'm missing [some or all] channels!!!
SOLUTION:
Some stations didn't quite make it clear on the 'hidden transition' and didn't tell all of us that they were switching around to VHF from UHF or vice versa, meaning the channel list in your box's memory is now obsolete, and all that needs to be done is to go to the menu on the box, and do a 'channel scan' and reset the channel list to update and you should see the channels re-appear, maybe even get some new ones. if you are still having trouble, see the 'Reception' forum here and ask someone. it may be in a location (like VHF) where it's hard to receive with your current antenna.