Typically, antenna problems will show themself's in the following manners, low or no rx, a quiet radio is not a good sign during the day, you should have some kind of traffic during daylight hours with the skip. low or no tx signal would mean the antenna isn't performing properly in some manner. Of course, no tx will also mean no power to the radio, a dead battery in a power mic ( but the radio itself will still show forward movement when you key up), if not, then something is wrong with the radio itself. the PA switch is on or broken, the meter is broken in the radio itself ( that's way I suggest using an external meter for all tests, the radio one is just to quick check that it's working before you hit the road, never rely on it for finish testing or true results).
(1) If the radio will not tx or rx and it gets power and other wise seems to be working , then take it to a shop and have them stick it on the bench to test it. They can tell in about 15 minutes how well the radio is working with out opening it. It would require opening it and checking the internals to find the fault if it indeed is the radio. If the radio checks out, then move to step 2.
(2) Check the coax and it's connectors, use a OHM meter on the coax disconnected from both the mount and the radio, bring the ends close enough so you can reach both of them. touch the 2 center pins and it should sound or show path. touch the center on one and the outside of the pl259 and there should be no sound or path, then touch the out side on both and it should sound or show path again, finally, touch centers and wiggle the coax ends to see if the problem is intermittent meaning a broken wire(s) of lose solider joint. Same with the outside, it the coax passes this test, move to the mount. Step 3.
(3) With the OHM meter, touch the tippy top of the mount and the center bottom pin of the SO239, it should show path, touch the tippy top and the side of the bottom below the mounting bracket, it should show no path. If your using a magnetic mount, this testing will not work, you need a different testing procedure which I will not go into because of the jumpers wires and second meter. If the mount checks out, go to step 4.
(4) was the antenna tuned once it was installed ? Did the setup work then all the sudden not work ? A shop can put the antenna on an analyzer and check the independence and load capabilities.
Rules to follow weather your running 4 watt's or 4000, ham, cb or fm...
Never, use Radio shack coax, they barely if ever solider the center pin of the coax and the ground braid is pinched into place, they should be soldiered. If you have to , buy some rg-8x clear from a shop with soldered PL259's on both ends. I use rg-8x clear, to visually inspect for copper rot in the wires. It will handle more then the normal cb rig puts out, up to 2000 watt's PEP.
Never use radio shack antennas. They suck, are expensive and un-reliable. The only reason to use them is no other way to get an antenna, even then, find a place where you can buy what you want, don't settle for what they have on stock, from anyone. Any shop can get you any antenna, it's just a matter of finding one in your area that will not rip you off and over charge you.
I might suggest borrowing someone else's radio that you know is working and try it in your rig, if it doesn't work in yours, you know it's the coax, mount or antenna, then follow the above mentioned methods. If it does work, then you know it's your radio, then follow what steps you need to have it repaired or replaced, if you paid 50 bucks for a radio, don't pay another 100.00 to fix it, save the money and buy another one. Just buy new with a warranty, you'll be happy later on if it fails in the warranty period.