Cannot receive fax ...

Many fax machines have little internal buffer memory and the memory serves to receive a paket of signals which arrive faster than can be printed but when it is full, it may pause so the printing portion can catch up.

Check your manual for the phrase 'buffer memory' to see if they discuss why and when this might happen.

It's possible that the machine is still on line and has just paused waiting for the buffer to reduce its contents.