My apps status suddenly become "not installed on the phone", even the google play store! I have been searching online for problem similar to my case but unfortunately didn't manage to find any solution to my case.
INFO
Samsung Galaxy S2 i9100T
droid version 4.03

STORY
I didn't do anything, never took out the SD card, or rooting or factory reset whatsoever all this time and my phone was working fine. But then today suddenly when I reached out for my phone to read one whatsapp message, some kind of error message shown up and need me to close it I think. I can't really remember what its saying, not just for whatsapp but along with several other apps.
Then I restarted my phone, and found in my home screen several shortcut icons missing, some are still there but with android logo. So I went to the main menu and noticed all the apps are there but it became gray colored with the same icon (Android with SD text on right corner).
When I pressed the app a message saying the app is not installed on the phone.
Another important thing is that I am unable to locate the google play store anywhere, even under the Setting-&gt;Apps-&gt;All. Actually, I can't find all the missing applications -- though in main menu I saw the gray colored play store icon (not google play but play store)
I don't think the SD card is the problem since my music and images are still there just fine, not to mention a few games apps still working fine even though they are also stored in SD.
edit:
Just now I tried to access google play store with internet browser on phone and tried to download the missing apps again but I can't, it gives the message your device already has this item. Even if I want to un-install the missing apps, I can't because I cannot locate them under Settings-&gt;Apps-&gt;All.
QUESTION
What do I need to do to fix this problem? I really have no idea since I am one that never really follows up with tech development of mobile phone, just take it and use it for granted.
I had the same problem. I fixed it by: 


shutting down my phone
removing the battery and SD card
putting it back 
turning my phone on. 


It's all there again.

If this does not work, try reinstalling the apps. 

Don't uninstall the apps with weird icon but go to the store and install them again.

No data will be lost!!! (Tried and tested)