Exposed abutment. What can I do? (Photo)

There is no issue with the abutment being exposed. It won't cause a problem. I agree with placing teflon tape in the hole. If food accumulates in the screw hole, it will make be difficult to fully clean out to get to the screw inside the abutment. Any dentist should be able to cement the crown back on assuming the implant and abutment are stable. 

There isn't a problem with temporarily sealing the abutment with teflon tape for a short duration, however, a better solution would be to try to get the crown recemented as soon as possible either temporarily or permanently.  The neighboring teeth and opposing teeth (on the upper arch) could shift and require the fabrication of a new crown.  Also, the gums could grow around the abutment and require lasering of the tissue before they can recement the crown.  I'd look for some temporary cement in a pharmacy if you don't have access to any dentists in your area.  If you do have access to dentists, I'd definitely make an appointment and get the crown recemented asap.