Have you still got Sonar 7 on your system?
 
The answer you provide will give us a clear way forward, no matter what the answer

Unfortunately, I don't have Cakewalk 7 installed on my computer. Can anyone think of a way to fix this issue? Thanks!

Ok, maybe I should have been a little clearer. 
 
Do you have access to any version prior to X1?
 
You'll need it in order to reinstall Pantheon.

3 Suggestions

• Reinstall the plug-in.
  You can reinstall the Sonar 7 plug-ins without reinstalling Sonar 7.
  Try installing it again & see if it fixes your issue.
• If you're using non-standard audio settings (i.e.  something other than 16 or 24 bit and 44.1k sample rate) try creating a project that does and see if you still have the issue.
• Contact Cakewalk tech support.
  Perhaps there was an update to Lexicon Pantheon post S7 that they can connect you up with.

And if none of those work, maybe you're just SOL (sorry); move on. 
 
 
  

Yes, it will need to be reinstalled from a pre-X series version of SONAR. Make sure to run the installer as administrator and install the appropriate version(s) (i.e 32bit for 32bit SONAR, 64bit for 64bit SONAR). It may also require SONAR to run as administrator to run the plug-in after it is reinstalled too.