Here are a few troubleshooting tips to try and fix your problem.
Is Everything Powered On?
A few years ago I exchanged multiple emails with a reader trying to troubleshoot why he had lost a signal. He had just bought an RF modulator and had done everything correctly in the situation. A week later, the person realized he hadn't switched on the RF modulator's power. We know you've checked already, but check again to be sure your converter box is getting power.
Is Everything Connected Properly?
Connecting a cable into the wrong port happens, which is why reviewing your connections is critical in helping determine the cause of signal loss. There are a couple of rules that can help when connecting cables. From source to display always connect output to input, and when possible match the colors at the end of the cable to the input. Make sure everything is matched up correctly and that the connections are secure.
Is Your TV Tuned to the Correct Channel and Correct Input Source?
Your TV should be tuned to channel 3 if the DTV converter box is connected to the TV with a coaxial cable. If you used a composite RCA cable, then you likely need to turn the TV to the AUX/Video channel. If the DTV converter box has a channel switch that changes between channels 3 and 4, then make sure you have it turned to the same channel that your TV is tuned to.
Did You Configure the DTV Converter Box Properly?
You must run a channel scan after connecting the DTV converter box. If you don't do a scan for channels, then your DTV converter box won't display any local channels. The scan is part of your DTV converter box's menu system, so use your remote control to access the menu and perform the scan.
Is the Antenna Aligned Properly or in the Best Location?
There are numerous problems associated with digital reception that are explained more deeply in an article about losing reception. For instance, broadcast towers might have changed locations, or the point on the tower at which the signal is transmitted from could be lower thus not traveling as far, or the frequency of the signal could have changed. Any of these factors can impact where your antenna should be installed and how it should be positioned.
This is the most difficult thing to troubleshoot with a DTV converter box. If you followed the previous steps, then you've already run another channel scan on the DTV converter box and are probably getting some sort of television signal. If you still don't have all of your channels -- even if one channel is missing -- then the source could very well be your antenna.
For outdoor antenna users, a site called AntennaWeb can make recommendations on the right antennae to use and the direction that the signals from different stations are coming from. We can help you understand how to use AntennaWeb's form. You'll be able to see how you need to align your antenna to get digital signals. It will also show you the best type of antenna for your area, so that you can tell if you even have the right antenna to begin with.
If you use an indoor antenna, then my best recommendation is to buy an antenna designed for digital reception -- especially if you currently use a directional antenna like rabbit ears. Antennas designed for digital are flat and should have amplification up to around 14db. The antenna needs to be multi-directional. An example of an antenna designed for digital reception is RCA's ANT1500.
Still stuck? Visit the TV Forums to get help with your specific problem.