If you purchased the software directly from Cakewalk try logging into the store and checking under "my account">View your products. See if you have more than one serial number for Sonar listed.
Then run sonar and check the Help>about page to see if the serial number matches what you are seeing in the Cakewalk store.
If there are multiple serial numbers or the one shown in the Sonar Application Help>about page you may need to try changing the serial number.
I do not know the procedure to do that. the only thing I can think of is a clean install.
Do not try this unless everything else has failed because I have not tested the following procedure This involves removing sonar using the windows control panel and then removing all the registry entries for sonar from the registry using regedit. If you search google for "sonar clean install" you should find the procedure for that from a previous version you will need to adjust a few things to accommodate the newer version.
If you are not an experienced windows user I would not attempt this without help as you could damage the registry if you are not careful.
Once sonar is removed reboot and install manually using the file in your Command center download folder. (Click the gear icon in the Command Center to see where that is)
During the install you will be asked for a serial number and you can put in the one you want to change to. Then after the instal completes you will need to follow the instruction for the offline install or you could try the online activation since you have now changed the serial number.
There may be other ways that are simpler but this is what I can think of to help you.