Python Thread dies silently

I am using a thread for running all time in my project, and it dies silently without any log.

Here is the code I'm using in and upload_manager module

logger = logging.getLogger('upload_manager')

class UploadManager:

_thread = None

@classmethod
def _run(cls):
    while cls._is_running:
        try:
            cls._process_queue()
            time.sleep(5)
        except Exception as ex:
            logger.exception(ex)
    cls._thread = None
    logger.warn("Upload Manager Thread is finished")

@classmethod
def start(cls):
    if cls._thread is not None:
        # Thread already exists.
        logger.warn("Uploading Manager Thread is already running.")
        return
    cls._thread = threading.Thread(name="uploader",
                                    target=cls._run,
                                    daemon=True)
    cls._is_running = True
    cls._thread.start()
    logger.info("Upload Manager Thread is started");

I start this thread from some other module like this:

    UploadManager.start()       

The Code ran for about two, three days and after that stopped silently. Can anybody help me with this issue?