FWIW, the NASA internal status report for 4/20
http://www.spaceref.com/news/viewsr.html?pid=20338 says
"ISS Reboost: The test of the SM main engines scheduled yesterday at
3:49pm EDT was aborted without engine ignition when the onboard
software received no signal that the sun cover on engine #2 had
opened."

If it is truly an external cover, this sounds like something that might
be repairable with an EVA (earlier reports implied the failure was a
propellant valve, which seems like it would be a lot nastier to work
on). Or for that matter, it might be a sensor failure, or an issue that
could be worked around with different procedures (random speculation:
opening the cover in a different thermal environment or giving the
actuator more current or more time to open.)

They also note that a reboost using only one of the SM main engine is
still possible.