I gotcha Sewn

Ok, there could be a number of problems for the blockage. Was the 901 in a fall recently? Doesn't really matter the question is irrelevant anyway as we're thinking of the problem and not the solution. What could be blocking maybe either a wire got jogged out of place, the internals were somehow pushed or well lubricated enough to sink or thicker liquid had a chance to solidify a bit while the atty was inactive for a period of time.
1st, let's try to be simple and wipe off the bottom of the atomizer and cover the air hole with a tissue and blow out through the bottom. If that fails we always have a step 2.
2.
Let's try taking a sewing

needle and inserting about 5-10mm into the air hole in a downward fashion. At this point your hand should be near the top of the atomizer, in a slow and careful way swing your hand down while holding the top of the needle to reach just under level of the air hole.
Let me know how it works out, please make the atomizer is still disconnected from the device while performing any of these steps. With Step 2 use caution on how deep you place the needle.