no air produced even though green light is on

The concentrator turns on, all three lights come on at first for a few seconds, then only green. However, there is no O2 being produced, the gauge stays at the bottom, no tubing or cannula attached.

Before this the red alarm light and sound came on after a few seconds and stayed on, so I replaced the filter. Now the concentrator has the above condition with the new filter and also with the old filter put back.

After running the concentrator for several minutes with the new filter (which does not seem to be the problem) the red light comes on and the alarm sounds continuously until I turn the machine off.