I'm not aware of any interaction between live MIDI input and ASIO buffer size. Re-starting SONAR will probably solve the problem, but if it happens again, start by splitting the Instrument track to see whether the problem is MIDI or audio.
If the synth has a virtual keyboard/controller or patch audition function, see if it generates output that way. If the audio engine didn't drop out completely when you lowered the buffer and audio track playback is working, you should be able to get output from the the synth directly.
If the audio side is working and the MIDI track is echoing input but the synth isn't responding, the most likely causes would be:
1. The synth is set to respond to a specific channel that doesn't match what your controller is sending.
2. The notes are outside the restricted range of the patch.
3. Some CC message has zeroed the output level (e.g. CC1=0 sent when Zero Controllers When Playback Stops is enabled in preferences will silence Garritan/ARIA Player).
4. The patch has an
extremely slow attack (possibly increased by an errant CC73 message) such that short duration notes don't reach an audible level (unlikely, but it has been known to happen).