orgtestngTestNGException Cannot find class in classpath

i am getting an error when running my script.Yestarday my code is working fine but today when i was running than getting an error .

unable to proceed further please help me on that..

its urgent...

Error: org.testng.TestNGException:
Cannot find class in classpath: com.xxxx.yyyy.abc
at org.testng.xml.XmlClass.loadClass(XmlClass.java:81)
at org.testng.xml.XmlClass.init(XmlClass.java:73)
at org.testng.xml.XmlClass.<init>(XmlClass.java:59)
at org.testng.xml.TestNGContentHandler.startElement(TestNGContentHandler.java:544)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractXMLDocumentParser.emptyElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.dtd.XMLDTDValidator.emptyElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)