what is the root cause of the following error:

Caused by: java.io.FileNotFoundException: File file: /nas/file123.csv does not exist
It is possible the underlying files have been updated. You can explicitly invalidate the cache in Spark by running 'REFRESH TABLE tableName command in SQL or by recreating the Dataset/DataFrame involved.
at org.apache.spark.sql.execution.datasources.FileScanDD$$anon$1.nextIterator(FileScanDD.scala:157)
at org.apache.spark.sql.execution.datasources.FileScanDD$$anon$1.hasNext(FileScanDD.scala:102)
at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIterator.processNext(UnknownSource)
at org.apache.spark.sql.execution. BufferedRowIterator.hasNext (BufferedRowIterator. java:43) at org.apache.spark.sql.execution.WholeStageCodegenExec$$anonfun$8$$anon$1.hasNext(WholeStageCodegenExec.scala:377) at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:408) at org.apache.spark.api.python.SerDeUtil$AutoBatchedPickler.hasNext(SerDeUtil.scala:117) at scala.collection.Iterator$class.foreach (Iterator.scala: 893)
at org.apache.spark.api.python.SerDeUtil$AutoBatchedPickler.foreach(SerDeUtil.scala:112)
at org.apache.spark.api.python.PythonRDD$.writeIteratorToStream(PythonRDD.scala:504)
at org.apache.spark.api.python. PythonRunner$WriterThread$$anonfun$run$3.apply(PythonRDD.scala:328) at org.apache.spark.util.Utils$.logUncaughtExceptions(Utils.scala:1951) at org.apache.spark.api.python.PythonRunner$WriterThread.run(PythonRDD.scala:269)