This can be caused by the Transaction Broker datalink pointing at the wrong text file or the file name for the target text file and actual text file not being the same. To check this;
Click the Transaction Broker menu and select Define Datalinks.
Select the correct datalink, click datalink items (magnifying glass after sheet with cross) icon.
In the Text File Location field, check the filepath and target filename to ensure that it is pointing to the correct file.
It may also be worth opening the file in Windows Explorer to ensure that it is not corrupt.
Note that if there are any errors from a previous run in the log then these must be cleared before you are able to change the text file name used by Transaction Broker.