Open up Comodo and go to D+/Advanced/Computer Security Policy. Find the program you blocked and right click on it and edit it. Make it trusted. Take time to learn what all the functions and tabs do in Comodo.
You can also look under the firewall. Go to Firewall/Advanced/Network Security Policy and edit what you blocked. Did you find what you blocked? Did you make it trusted and click apply?
Try to do what Vet said but “Remove” the appropriate entry.This way Defence+ should learn the application again.
Alternatively you may have blocked explorer from running it.
From Computer Security Policy find the %windir%\explorer.exe entry,highlight it and click"Edit" now click “Modify” next to run an executable.You should get a new window with allowed and blocked applications,look under the blocked applications list to see if anything is there.If it is “Remove” it and APPLY to close all windows.