Hi
I just came across your post
This may be too late to help, but here goes
Install Midi monitor MIdi-OX on your PC (free)
With this software running, if you can see midi messages coming into your PC from the SL990
then its definitely not a problem with the SL990 and it will also confirm no problems with your Ploytec.
Midi messages show up as green dots for Midi In data.
If no midi messages in are seen
Is cable in Midi Out socket on SL990
Is Ploytec cable plugged in at the PC end, if its a USB then is the port configured on the PC
If the Ploytec is USB (I don't know the brand) be sure to use exactly the same USB port on your PC as the one used when the SL90 was 'working yesterday'
Just checked out the Ploytec website, that gear looks complex
Have you tried using a standard midi-USB converter, eg Roland UM-1. simpler.