I'm going to assume you've checked the obvious things, such as reviewing the manual, checking settings, and inspecting equipment, cables, connectors, and grounds. If not, start there.
After that, start by breaking the problem down into sections.
There's 3 broad possibilities: there's no radio signals to receive, or there's a problem with the antenna/feedline, or there's a problem with the radio.
Start with the simplest possibility first, then proceed towards the most complicated.
First, test for signals. Get a friend over who has a CB in his vehicle, or borrow a handheld CB, or whatever, and tune your radio and the borrowed radio to the same channel at the same time. If you can hear traffic on a channel with the borrowed radio, but not with your radio, then you know the problem lies with your equipment somehow.
Second, determine if the problem lies with the antenna/feedline assembly, or the radio. Do one of the following, whatever is easiest: either connect a borrowed mobile radio to your power, feedline, and antenna; or connect a borrowed antenna and feedline to your radio; or remove your antenna and feedline and connect it to another radio; or pull your radio and connect it to power, feedline, and antenna in another vehicle.
Pay attention to which parts the problem follows. If a borrowed radio works with your antenna/feedline, then your radio has a problem. If a borrowed antenna/feedline works with your radio, then your antenna/feedline has a problem.
If it's your antenna/feedline, swap just the antenna or feedline until you determine which is good and bad, then either figure out why or replace the bad part. If it's the radio, either get a new one, or pull the cover off and inspect the circuit boards and internal wires and connectors. Note any discolored circuit boards or components, or loose connectors or discolored or broken wires.
Let us know the results.