When I close X2 and want to start it up again after a while, nothing happens. I need to restart my PC to start X2 again.
Can anybody explain this and help me to solve this issue?

Perhaps there is a driver that prevents SONAR from closing fully?  Look for a window hidden behind SONAR.
 
Also, whatever the cause, can you use Task Manager to close SONAR, instead of having to reboot?  It does make sense that you would not want to start a second instance of SONAR.  [I don't recall if this is even allowed, and I'm not at my SONAR computer to check in the preferences.]

This happens to me too. Has from the beginning. I just re-boot.

Thanks Riojazz. Thanks to you I discovered in my Windows Taskmanager under the process tab, that both bitbridge.exe and sonarpdr.exe were still active in the background! So even when I end my X2-session/program in a normal way (by clicking the cross in the right upper corner), the program does not really close, so it is impossible to start it up again, unless I end up the process of sonarpdr.exe in the taskmanager!

It may shutdown if you cycle the power to your audio interface (if possible). As riojazz indicated, SONAR is probably waiting on the audio driver.

So, Bitbridge running means you have 32-bit plugins loaded into your project, and it is my observation that the usual cause of that kind of error is that one or more of those 32-bit plugins does not play well in 64-bit Sonar.
 
You will likely have to evaluate the ones you have loaded and rethink them.  Finding the culprit or culprits is a royal pain.  I would start with freeware 32-bit plugins, as these are usually not as robustly coded - though this is not always the case.
 
Anyways, if you can find and eliminate the one or ones that are wrecking your ability to shut down, you should be OK thereafter.  You can always install the 32-bit Sonar if you absolutely need to work with those particular plugins.
 
Bob Bone
 

Hmmm...Interesting. It hasn't really bothered me much, but it's nice to know the answer. Learn something new everyday.

You are right Bob, I started a new blank project without any plugins, and I could close and start X2 as often as I wanted! So now I will find out what plugin causes the problem.
 
Thanks a lot guys,
didap

I just had this happen yesterday. My Softube mix bundle was the culprit. I uninstalled it and reinstalled it and it's been working fine since.

When I discovered this issue, I immediately put the brakes on using 32-bit plugins in 64-bit Sonar, UNTIL I went through the exercise of determining which ones worked OK and which one did not.
 
Once I had the list of ones I was comfortable with using in my 64-bit Sonar install, I moved those 32-bit plugins to a special folder within my C:\\Program Files (x86)\VST Plugins folder, as a SUB-folder called '32-Bit Cleared'.  (the name is of course arbitrary).  I then put that path into the x64-bit Sonar search path for VST plugins in the Cakewalk Plug-in Manager, and then ONLY those 32-bit plugins that I had determined would work properly in 64-bit Sonar were made available for choice in projects.  (this new path REPLACED my old path that had pointed to the entire 32-bit VST Plugins folder).
 
There are many ways to do things, but I found this to work for me, and it keeps the paths simple for both 32-bit and 64-bit Sonar installs.  By keeping those plugins in a sub-folder in the C:\\Program Files (x86)\VST Plugins folder, the 32-bit Sonar still finds them, and by ONLY having that extended path of 'C:\\Program Files (x86)\VST Plugins\32-bit Cleared' in the 64-bit Sonar, everything is peachy keen.
 
I now NEVER have crashes with my 64-bit X2a install, even when I do use 32-bit plugins, because I already know that only those that work are made available to it.
 
Hope that helps, 
 
Bob Bone