There are a number of processes triggered by PD running in the background. After closing down PD, it can happen that one or more of these processes or even PD itself remains active in the background. In that case PD will not open again. Normally it will give a message saying that only one instance can be activated. I have not seen yet the balloon help (??) that stays on screen. Can you share a screenshot?
I trust that you have done all the obvious things:

• look at PD/CL processes running in the background and kill them
• Restart windows (assuming that is what you use)?
• uninstall PD and reinstalled it?
• Look at all your drivers and make sure that they are up to date.

If not I suggest that you make a screen shot and share that with CL customer support, since they can help you better with these kind of flaws than we can here.
It will also help if you share the operating system and version nr of PD and even a DXDiag.txt file.