HTC Wildfire S: anybody help pls???

Hi folks i`m new to the forum so hope ive put this in the correct place .what it is i`m not to clever with mobile phones and was wondering if anyone can help me here ??? the question is i looked at my htc wildfire phone tonight and on the screen its got "limited service" on it and emergency calls only ` can anybody tell me what it is also the bar where my signal shows up is usualy green bars but theres nothing there at all ,is this something about my signal ??? would be much obligded for any help .

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