QLn320 can receive print data but does not print

I am developing iOS app with QLn320 printer. The app could print in the last two months, but from yesterday the printer stops printing. The sample app (Signature Capture) in the SDK cannot print either. So I think it should not be related with my code.

The symptom is: the Data icon on the printer flashes for multiple times and then stops flashing, but paper is not fed and nothing is printed.

When my iOS app sends print command, the printer screen becomes bright while the data icon is flashing.

Is there something wrong with the printer?